trunk/doc/syntax.lyx

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\author "" 
\end_header

\begin_body

\begin_layout Chapter
Syntax reference
\begin_inset LatexCommand label
name "cha:ASCENDSyntax"

\end_inset


\end_layout

\begin_layout Standard
We shall present an informal description of the ASCEND IV language.
 Being informal, we shall usually include examples and descriptions of the
 intended semantics along with the syntax of the items.
 At times the inclusion of semantics will seem to anticipate later definitions.
 We do this because we would also like this chapter to be used as a reference
 for the ASCEND language even after one generally understands it.
 Often one will need to clarify a point about a particular item and will
 not wish to have to search in several places to do so.
\end_layout

\begin_layout Standard
Syntax
\begin_inset LatexCommand index
name "syntax"

\end_inset

 is the form or structure for the statements in ASCEND, where one worries
 about the exact words one uses, their ordering, the punctuation, etc.
 Semantics
\begin_inset LatexCommand index
name "semantics"

\end_inset

 describe the meaning of a statement.
\end_layout

\begin_layout Standard
To distinguish between syntax and semantics, consider the statement
\end_layout

\begin_layout LyX-Code
y IS_A fraction;
\end_layout

\begin_layout Standard
Rules on the syntax for this statement tell us we need a user supplied instance
 name, y, followed by the ASCEND operator IS_A, followed by a type name
 (fraction).
 The statement terminates with a semicolon.
 The statement semantics says we are declaring the existence of an instance,
 locally named y, of the type fraction as a part within the current model
 definition and it is to be constructed when an instance of the current
 model definition is constructed.
\end_layout

\begin_layout Standard
The syntax for a computer language is often defined by using a Bachus-Naur
\begin_inset LatexCommand index
name "Bachus-Naur"

\end_inset

 formal (BNF
\begin_inset LatexCommand index
name "BNF"

\end_inset

) description.
 The complete YACC
\begin_inset LatexCommand index
name "YACC"

\end_inset

 and FLEX
\begin_inset LatexCommand index
name "FLEX"

\end_inset

 description of the language described (as presently implemented) is available
 by FTP and via the World Wide Web.
 The semantics of a very high level modeling language such as ASCEND IV
 are generally much more restrictive than the syntax.
 For this reason we do not include a BNF description in this paper.
 ASCEND IV is an experiment.
 The language is under constant scrutiny and improvement, so this document
 is under constant revision.
 
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
\InsetSpace ~

\end_layout

\begin_deeper
\begin_layout Chapter
Preliminaries
\end_layout

\begin_layout Standard
We will start off with some background information and some tips that make
 the rest of the chapter easier to read.
 ASCEND is an object-oriented (OO) language for hierarchical modeling that
 has been somewhat specialized for mathematical models.
 Most of the specialization is in the implementation and the user interface
 rather than the language definition.
 
\end_layout

\begin_layout Standard
We feel the single most distinguishing feature of mathematical models is
 that solving them efficiently requires that the solving algorithms be able
 to address the entire problem either simultaneously or in a decomposition
 of the natural problem structure that the algorithm determines is best
 for the machine(s) in use.
 In the ASCEND language object-orientation is used to organize natural structure
s and make them easier to understand.
 It is not used to hide the details of the objects.
 The user (or machine) is free to ignore uninteresting details, and the
 ASCEND environment provides tools for the runtime suppression of these.
\end_layout

\begin_layout Standard
ASCEND is well into its 4th generation.
 Some features we will describe are not yet implemented (some merely speculative
) and these are clearly marked (* 4+ *).
 Any feature not marked (* 4+ *)has been completely implemented, and thus
 any mismatch between the description given here and the software we distribute
 is a bug we want you to tell us about.
 
\end_layout

\begin_layout Standard
The syntax and semantics of ASCEND may seem at first a bit unusual.
 However, do not be afraid to just try what comes naturally if what we write
 here is unclear.
 The parser and compiler of ASCEND IV really will help you get things right.
 Of course if what we write here is unclear, please ask us about it because
 we aim to continuously improve both this document and the language system
 it describes.
\end_layout

\begin_layout Standard
We will describe, starting in Section 
\begin_inset LatexCommand vref
reference "sub:x.1.2Basic-Elements"

\end_inset

, the higher level concepts of ASCEND, but first some important punctuation
 rules.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
ASCEND\InsetSpace ~
is\InsetSpace ~
cAsE\InsetSpace ~
sensitive
\begin_inset LatexCommand index
name "case sensitive"

\end_inset

! 
\end_layout

\begin_deeper
\begin_layout Standard
The keywords that are shown capitalized (or in lower case) in this chapter
 are that way because ASCEND is case sensitive.
 IS_A is an ASCEND keyword; isa, Is_a, and all the other permutations you
 can think of are NOT equivalent to IS_A.
 In declaring new types of models and variables the user is free to use
 any style of capitalization he or she may prefer; however, they must remain
 consistent or undefined types and instances will result.
\end_layout

\begin_layout Standard
This case restriction makes our code very readable, but hard to type without
 a smart editor.
 We have kept the case-sensitivity because, like all mathematicians, we
 find ourselves running out of good variable names if we are restricted
 to a 26 letter alphabet.
 We have developed smart add-ins for two UNIX editors, EMACS
\begin_inset LatexCommand index
name "EMACS"

\end_inset

 and vi
\begin_inset LatexCommand index
name "vi"

\end_inset

, for handling the upper case keywords and some other syntax elements.
 The use of these editors is described in another chapter.
 
\end_layout

\begin_layout Standard
The ASCEND IV parser is very picky and pedantic.
 It also tries to give helpful messages and occasionally even suggestions.
 New users should just dive in and make errors, letting the system help
 them learn how to avoid errors.
\end_layout

\begin_layout Section
Punctuation
\begin_inset LatexCommand index
name "punctuation"

\end_inset


\end_layout

\end_deeper
\begin_layout Standard
This section covers both the punctuation that must be understood to read
 this document and the punctuation of ASCEND code.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
keywords
\begin_inset LatexCommand index
name "keywords"

\end_inset

: ASCEND keywords and type names are given in the left column in bold format.
 It is generally clear from the main text which are keywords and which are
 type names.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Minor\InsetSpace ~
items:
\bar default
 Minor headings that are helpful in finding details are given in the left
 column in underline format.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
Tips: Special notes and hints are sometimes placed on the left.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
LHS
\begin_inset LatexCommand index
name "LHS"

\end_inset

:
\bar default
 Left Hand Side.
 Abbreviation used frequently.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
RHS
\begin_inset LatexCommand index
name "RHS"

\end_inset

:
\bar default
 Right Hand Side.
 Abbreviation used frequently.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Simple\InsetSpace ~
names
\begin_inset LatexCommand index
name "simple names"

\end_inset

:
\bar default
 In ASCEND simple names are made of the characters a through z, A through
 Z, _, (*4+*: $).
 The underscore is used as a letter, but it cannot be the first letter in
 a name.
 The $
\begin_inset LatexCommand index
name "\\$"

\end_inset

 character is used exclusively as the first character in the name of system
 defined built-in parts.
 "$" is explained in more detail in Section 
\begin_inset LatexCommand vref
reference "sub:x.6.2Supported-attributes"

\end_inset

.
 Simple names should be no more than 80 characters long.
 
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Compound\InsetSpace ~
names
\begin_inset LatexCommand index
name "compound names"

\end_inset

:
\bar default
 Compound names are simple names strung together with dots (.).
 See the description of "." below.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Groupings:
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
«\InsetSpace ~
»
\bar default
 In documentation 
\series bold
optional fields
\series default

\begin_inset LatexCommand index
name "optional fields"

\end_inset

 are surrounded by these markers.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
(*\InsetSpace ~
*)
\family default

\begin_inset LatexCommand index
name "(* *)"

\end_inset

 Comment.
 Anything inside these is a comment.
 Comments can nest
\begin_inset LatexCommand index
name "nest"

\end_inset

 in ASCEND and span several lines.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
(\InsetSpace ~
)
\family default

\begin_inset LatexCommand index
name "( )"

\end_inset

 Rounded parentheses
\begin_inset LatexCommand index
name "parentheses"

\end_inset


\begin_inset LatexCommand index
name "rounded parentheses"

\end_inset

.
 Used to enclose arguments for functions or models where the order of the
 arguments matters.
 Also used to group terms in complex arithmetic, logical, or set expressions
 where the order of operations needs to be specified.
 
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
Efficiency\InsetSpace ~
tip: The compiler can simplify relation definitions in a particularly
 efficient manner if constants are grouped together.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
{\InsetSpace ~
}
\family default

\begin_inset LatexCommand index
name "{ }"

\end_inset

 Curly braces
\begin_inset LatexCommand index
name "Curly braces"

\end_inset

.
 Used to enclose units.
 For example, 1 {kg_mole/s}.
 Also used to enclose the body of annotations.
 Note: Curly braces are also used in TCL, the language of the ASCEND user
 interface, about which we will say more in another chapter.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
[\InsetSpace ~
]
\family default

\begin_inset LatexCommand index
name "[ ]"

\end_inset

 Square brackets
\begin_inset LatexCommand index
name "square brackets"

\end_inset

.
 Used to enclose sets or elements of sets.
 Examples: my_integer_set :== [1,2,3], demonstrates the use of square brackets
 in the assignment of a set.
 My_array[1] demonstrates the use of square brackets in naming an array
 object indexed over an integer set which includes the element 1.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
.
\family default

\begin_inset LatexCommand index
name "."

\end_inset

 Dot
\begin_inset LatexCommand index
name "dot"

\end_inset

.
 The dot is used, as in PASCAL and C, to construct the names of nested objects.
 Examples: if object a has a part b, then the way to refer to b is as a.b.
 Tray[1].vle shows a dot following a square bracket; here Tray[1] has a part
 named vle.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
..
\family default

\begin_inset LatexCommand index
name ".."

\end_inset

 Dot-dot or double dot
\begin_inset LatexCommand index
name "double dot"

\end_inset

.
 Integer range shorthand.
 For example, my_integer_set :== [1,2,3] and my_integer_set :== [1..3] are
 equivalent.
 If ..
 appears in a context requiring (), such as the ALIASES/IS_A statement,
 then the range is expanded and ordered as we would naturally expect.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
:
\family default

\begin_inset LatexCommand index
name ":"

\end_inset

 Colon
\begin_inset LatexCommand index
name "colon"

\end_inset

.
 A separator used in various ways, principally to set the name of an arithmetic
 relation apart from the definition.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
::
\family default

\begin_inset LatexCommand index
name "::"

\end_inset

 Double colon
\begin_inset LatexCommand index
name "double colon"

\end_inset

.
 A separator used in the methods section for accessing methods defined on
 types other than the type the method is part of.
 Explained in Section 
\begin_inset LatexCommand vref
reference "sec:x.4Procedural-statements"

\end_inset

.[
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
;
\family default

\begin_inset LatexCommand index
name ";"

\end_inset

 Semicolon
\begin_inset LatexCommand index
name "semicolon"

\end_inset

.
 The separator of statements.
\end_layout

\begin_deeper
\begin_layout Section
Basic Elements
\begin_inset LatexCommand index
name "basic elements"

\end_inset


\begin_inset LatexCommand label
name "sub:x.1.2Basic-Elements"

\end_inset


\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Boolean\InsetSpace ~
value
\bar default

\begin_inset LatexCommand index
name "value, Boolean"

\end_inset


\begin_inset LatexCommand index
name "Boolean value"

\end_inset

 
\family sans
TRUE
\family default

\begin_inset LatexCommand index
name "TRUE"

\end_inset

 or 
\family sans
FALSE
\family default

\begin_inset LatexCommand index
name "FALSE"

\end_inset

.
 Can't get much simpler, eh? In the language definition 
\family sans
TRUE
\family default
 and 
\family sans
FALSE
\family default
 do not map to 1 and 0 or any other type of numeric value.
 (In the implementation, of course, they do.)
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
User\InsetSpace ~
interface\InsetSpace ~
tip: The ASCEND user interface programmers have found it very
 convenient, however, to allow T/F, 1/0, Y/N, and other obvious boolean
 conventions as interactive input when assigning boolean values.
 We are lazy users.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Integer\InsetSpace ~
value
\bar default

\begin_inset LatexCommand index
name "value, integer"

\end_inset


\begin_inset LatexCommand index
name "integer value"

\end_inset

 A signed whole number up to the maximum that can be represented by the
 computer on which one is running ASCEND.
 
\family sans
MAX_INTEGER
\family default

\begin_inset LatexCommand index
name "MAX\\_INTEGER"

\end_inset

 is machine dependent.
 Examples are:
\end_layout

\begin_deeper
\begin_layout LyX-Code
\align block
123
\end_layout

\begin_layout LyX-Code
\align block
-5
\end_layout

\begin_layout LyX-Code
\align block
MAX_INTEGER, typically 2147483647.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Real\InsetSpace ~
value
\bar default

\begin_inset LatexCommand index
name "value. real"

\end_inset


\begin_inset LatexCommand index
name "real value"

\end_inset

 ASCEND represents reals almost exactly as any other mathematically oriented
 programming language does.
 The mantissa
\begin_inset LatexCommand index
name "mantissa"

\end_inset

 has an optional negative sign followed by a string of digits and at most
 one decimal point.
 The exponent is the letter 
\family typewriter
e
\family default
 or 
\family typewriter
E
\family default
 followed by an integer.
 The number must not exceed the largest the computer is able to handle.
 There can be no blank characters in a real.
 
\family sans
MAX_REAL
\family default

\begin_inset LatexCommand index
name "MAX\\_REAL"

\end_inset

 is machine dependent.
 The following are legitimate reals in ASCEND: 
\end_layout

\begin_deeper
\begin_layout LyX-Code
\align block
-1
\end_layout

\begin_layout LyX-Code
\align block
1.2
\end_layout

\begin_layout LyX-Code
\align block
1.3e-2 
\end_layout

\begin_layout LyX-Code
\align block
7.888888e+34
\end_layout

\begin_layout LyX-Code
\align block
.6E21
\end_layout

\begin_layout LyX-Code
\align block
MAX_REAL, typically about 1.79E+308.
\end_layout

\begin_layout Standard
while the following are not:
\end_layout

\begin_layout LyX-Code
\align block
1.
 2    (*contains a blank within it*)
\end_layout

\begin_layout LyX-Code
\align block
1.3e2.0 (*exponent has a decimal in it*)
\end_layout

\begin_layout LyX-Code

\family typewriter
+1.3    (* illegal unary + sign.
 x = +1.3 not allowed*)
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Reals\InsetSpace ~
stored\InsetSpace ~
in\InsetSpace ~
SI
\begin_inset LatexCommand index
name "SI"

\end_inset

\InsetSpace ~
units
\bar default
 
\end_layout

\begin_deeper
\begin_layout Standard
We store all real values as double precision
\begin_inset LatexCommand index
name "double precision"

\end_inset

 numbers in the metre-kilogram-second (MKS)
\begin_inset LatexCommand index
name "MKS"

\end_inset

 system of units.
 This eliminates many common errors in the modeling of physical systems.
 Since we also place the burden of scaling equations on system routines
 and a simple modeling methodology, the internal units are not of concern
 to most users.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Dimensionality
\begin_inset LatexCommand index
name "dimensionality"

\end_inset

:
\bar default
 Real values have dimensionality such as length/time for velocity.
 
\series bold
Dimensionality
\series default
 is to be distinguished from the 
\series bold
units
\series default
 such as ft/s.
 ASCEND takes care of mapping between units
\begin_inset LatexCommand index
name "units"

\end_inset

 and dimensions.
 A value without units (this includes integer values) is taken to be dimensionle
ss
\begin_inset LatexCommand index
name "dimensionless"

\end_inset

.
 Dimensionality is built up from the following base dimensions:
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
Name 
\bar under
definition; typical units
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
L
\begin_inset LatexCommand index
name "L, length dimension"

\end_inset

 length
\begin_inset LatexCommand index
name "length"

\end_inset

; metre, m
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
M
\begin_inset LatexCommand index
name "M, mass dimension"

\end_inset

 mass
\begin_inset LatexCommand index
name "mass"

\end_inset

; kilogram, kg
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
T
\begin_inset LatexCommand index
name "T, time dimension"

\end_inset

 time
\begin_inset LatexCommand index
name "time"

\end_inset

; second, s
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
E
\begin_inset LatexCommand index
name "E, electric current dimension"

\end_inset

 electric current
\begin_inset LatexCommand index
name "electric current"

\end_inset

; ampere, A
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
Q
\begin_inset LatexCommand index
name "Q, quantity dimension"

\end_inset

 quantity
\begin_inset LatexCommand index
name "quantity"

\end_inset

; mole, mole
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
TMP
\begin_inset LatexCommand index
name "TMP, temperature dimension"

\end_inset

 temperature
\begin_inset LatexCommand index
name "temperature"

\end_inset

; Kelvin, K
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
LUM
\begin_inset LatexCommand index
name "LUM, luminous intensity dimension"

\end_inset

 luminous intensity
\begin_inset LatexCommand index
name "luminous intensity"

\end_inset

; candela, cd
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
P
\begin_inset LatexCommand index
name "P, phase angle dimension"

\end_inset

 plane angle
\begin_inset LatexCommand index
name "plane angle"

\end_inset

; radian, rad
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
S
\begin_inset LatexCommand index
name "S, solid angle dimension"

\end_inset

 solid angle
\begin_inset LatexCommand index
name "solid angle"

\end_inset

; steradian, srad
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
C
\begin_inset LatexCommand index
name "C, currency dimension"

\end_inset

 currency
\begin_inset LatexCommand index
name "currency"

\end_inset

; currency, CR
\end_layout

\begin_deeper
\begin_layout Standard
The atom and constant definitions in the library illustrate the use of dimension
ality.
\end_layout

\begin_layout Standard
Dimensions may be any combination of these symbols along with rounded parenthese
s, (), and the operators *, 
\family typewriter
^
\family default
 and 
\family typewriter
/
\family default
.
 Examples include 
\family typewriter
M/T
\family default
 or 
\family typewriter
M*L^2/T^2/TMP
\family default
 {this latter means 
\family typewriter
(M*(L^2)/(T^2))/TMP
\family default
}.
 The second operand for the to-the-power-of operator, 
\family typewriter
^
\family default
, must be an integer value (e.g., -2 or 3) because fractional powers of dimensiona
l numbers are physically undefined 
\begin_inset Note Note
status open

\begin_layout Standard
although we do seem them occasionally...
\end_layout

\end_inset

.
\end_layout

\begin_layout Standard
If the dimensionality for a real value is undefined, then ASCEND gives it
 a wildcard dimensionality
\begin_inset LatexCommand index
name "wild card dimensionality"

\end_inset

.
 If ASCEND can later deduce its dimensionality from its use in a model definitio
n it will do so.
 For example consider the real variable 
\family typewriter
a
\family default
, suppose 
\family typewriter
a
\family default
 has wildcard dimensionality, 
\family typewriter
b
\family default
 has dimensionality of 
\family typewriter
L/T
\family default
.
 Then the statement:
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Example\InsetSpace ~
of\InsetSpace ~
a\InsetSpace ~
dimensionally\InsetSpace ~
consistent
\begin_inset LatexCommand index
name "dimensionally consistent"

\end_inset

\InsetSpace ~
equation.
 
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
\InsetSpace ~
 
\family typewriter
a + b = 3 {ft/s};
\end_layout

\begin_deeper
\begin_layout Standard
requires that 
\family typewriter
a
\family default
 have the same dimensionality as the other two terms, namely, 
\family typewriter
L/T
\family default
.
 ASCEND will assign this dimensionality to 
\family typewriter
a
\family default
.
 The user will be warned of dimensionally inconsistent equations.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Unit\InsetSpace ~
expression
\bar default
 A unit expression
\begin_inset LatexCommand index
name "unit expression"

\end_inset

 may be composed of any combination of unit names defined by the system
 and any numerical constants combined with times (
\family typewriter
*
\family default
), divide(
\family typewriter
/
\family default
) and to the power (
\family typewriter
^
\family default
) operators.
 The RHS of 
\family typewriter
^
\family default
 must be an integer.
 Parentheses can be used to group subexpressions with the exception that
 a divide operator may 
\emph on
not
\emph default
 be followed by a grouped subexpression.
 
\end_layout

\begin_deeper
\begin_layout Standard
So, 
\family typewriter
{kg/m/s}
\family default
 is fine, but 
\family typewriter
{kg/(m*s)}
\family default
 is not.
 Although the two expressions are mathematically equivalent, it makes the
 system programming and output formatting easier to code and faster to execute
 if we disallow expressions of the latter sort.
\end_layout

\begin_layout Standard
The units understood by the system are defined in the first part of this
 manual.
 Note that several units defined are really values of interesting constants
 in SI, e.g.
 
\family typewriter
R :== 1{GAS_C}
\family default
 yields the correct value of the thermodynamic gas constant.
 Users can define additional units.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Units
\bar default

\begin_inset LatexCommand index
name "units"

\end_inset

 A Unit expression unit expression must be enclosed in curly braces 
\family typewriter
{}
\family default
.
 When a real number is used in a mathematical expression in ASCEND, it must
 have a set of units expressed with it.
 If it does not, ASCEND assumes the number is dimensionless, which may not
 be the intent of the modeler.
 An example is shown in the dimensionally consistent equation above where
 the number 3 has the units 
\family typewriter
{ft/s}
\family default
 associated with it.
 
\end_layout

\begin_deeper
\begin_layout Standard
Examples:
\end_layout

\begin_layout LyX-Code
\align block
{kg_mole/s/m} same as {(kg_mole/s)/m}
\end_layout

\begin_layout LyX-Code
\align block
{m^3/yr}
\end_layout

\begin_layout LyX-Code
\align block
{3/100*ft} same as {0.03*ft}
\end_layout

\begin_layout LyX-Code
\align block
{s^-1} same as {1/s}
\end_layout

\begin_layout Standard
Illegal unit examples are
\end_layout

\begin_layout LyX-Code
\align block
{m/(K*kg_mole)}
\end_layout

\begin_deeper
\begin_layout Standard
grouped subexpression used in the denominator; should be written {m/K/kg_mole}.
\end_layout

\end_deeper
\begin_layout LyX-Code
\align block
{m^3.5}
\end_layout

\begin_deeper
\begin_layout Standard
power of units or dimensions must be integer.
\end_layout

\end_deeper
\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Symbol\InsetSpace ~
Value
\bar default

\begin_inset LatexCommand index
name "value, symbol"

\end_inset


\begin_inset LatexCommand index
name "symbol value"

\end_inset

 The format for a symbol is that of an arbitrary character string enclosed
 between two single quotes.
 There is no way to embed a single quote
\begin_inset LatexCommand index
name "'"

\end_inset


\begin_inset LatexCommand index
name "single quote"

\end_inset

 in a symbol: we are not in the escape sequence business at this time.
 The following are legal symbols in ASCEND:
\end_layout

\begin_deeper
\begin_layout LyX-Code
'H2O'
\end_layout

\begin_layout LyX-Code
'r1'
\end_layout

\begin_layout LyX-Code
'Bill said,foo to whom?'
\end_layout

\begin_layout Standard
while the following are not legal symbol values:
\end_layout

\begin_layout LyX-Code
"ethanol" (double quotes not allowed)
\end_layout

\begin_layout LyX-Code
water (no single quotes given)
\end_layout

\begin_layout LyX-Code
'i can't do this' (no embedded quotes)
\end_layout

\begin_layout Standard
There is an arbitrary upper limit to the number of characters in a symbol
 (something like 10,000) so that we may detect a missing close quote in
 a bad input file without crashing.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Sets\InsetSpace ~
values
\begin_inset LatexCommand index
name "values, set"

\end_inset


\begin_inset LatexCommand index
name "set values"

\end_inset

 Set values are lists of elements, all of type 
\family sans
integer_constant
\family default
 or all of type 
\family sans
symbol_constant
\family default
, enclosed between square brackets 
\family typewriter
[]
\family default
.
 The following are examples of sets:
\end_layout

\begin_deeper
\begin_layout LyX-Code
\align block
['methane', 'ethane', 'propane']
\end_layout

\begin_layout LyX-Code
\align block
[1..5, 7, 15]
\end_layout

\begin_layout LyX-Code
\align block
[2..n_stages]
\end_layout

\begin_layout LyX-Code
\align block
[1, 4, 2, 1, 16]
\end_layout

\begin_layout LyX-Code
\align block
[]
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
More\InsetSpace ~
about\InsetSpace ~
sets
\begin_inset LatexCommand index
name "sets"

\end_inset

\InsetSpace ~
in\InsetSpace ~
Section\InsetSpace ~

\begin_inset LatexCommand vref
reference "sub:x.2.2Sets"

\end_inset

.
\end_layout

\begin_deeper
\begin_layout Standard
The value range 
\family typewriter
1..5
\family default
 is an allowable shorthand for the integers 1, 2, 3, 4 and 5 while the value
 range 
\family typewriter
2..n_stages
\family default
 (where 
\family typewriter
n_stages
\family default
 must be of type 
\family sans
integer_constant
\family default
) means all integers from 2 to 
\family typewriter
n_stages
\family default
.
 If 
\family typewriter
n_stages
\family default
 is less than 2, then the third set is empty.
 The repeated occurrence of 
\family typewriter
1
\family default
 in the fourth set is ignored.
 The fifth set is the empty set
\begin_inset LatexCommand index
name "empty set"

\end_inset


\begin_inset LatexCommand index
name "set, empty"

\end_inset

.
\end_layout

\begin_layout Standard
We use the term set in an almost pure mathematical sense.
 The elements have no order.
 One can only ask two things of a set: (1) if an element is a member of
 it and (2) its cardinality
\begin_inset LatexCommand index
name "cardinality"

\end_inset

 (
\family sans
CARD
\family default

\begin_inset LatexCommand index
name "CARD"

\end_inset


\family typewriter
(set)
\family default
).
 Repeated elements used in defining a set are ignored.
 The elements of sets cannot themselves be sets in ASCEND; i.e., there can
 be no sets of set.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Sets\InsetSpace ~
are\InsetSpace ~
unordered.
 A set of integers may appear to be ordered to the modeler as the natural
 numbers have an order.
 However, it is the user imposing and using the ordering, not ASCEND.
 ASCEND sees these integers as elements in the set with NO ordering.
 Therefore, there are no operators in ASCEND such as successor or precursor
 member of a set.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Arrays
\bar default

\begin_inset LatexCommand index
name "arrays"

\end_inset

 An array is a list of instances indexed over a set, in computer-speak,
 an associative array of objects.
 The instances are all of the same base type (as that is the only way they
 can be defined).
 An individual member of a list may later be more refined than the other
 members (we shall illustrate that possibility).
 The following are arrays in ASCEND.
\end_layout

\begin_deeper
\begin_layout LyX-Code
\align block
stage[1..n_stages]
\end_layout

\begin_layout LyX-Code
\align block
y[components]
\end_layout

\begin_layout LyX-Code
\align block
column[areas][processes]
\end_layout

\begin_layout Standard
where components, areas and processes are sets.
 For example components could be the set of symbols 
\family typewriter
['ethylene', 'propylene']
\family default
, areas the set of symbols 
\family typewriter
['feed_prep', 'prod_purification']
\family default
 while processes could be the set 
\family typewriter
['alcohol_manuf', 'poly_propropylene_manuf']
\family default
.
 Note that the third example (
\family typewriter
column
\family default
) is a list of lists (the way that ASCEND permits a multiply subscripted
 array).
 
\end_layout

\begin_layout Standard
The following are elements in the above arrays:
\end_layout

\begin_layout LyX-Code
stage[1] 
\end_layout

\begin_layout LyX-Code
y['ethylene']
\end_layout

\begin_layout LyX-Code
column['feed_prep'][alcohol_manuf']
\end_layout

\begin_layout Standard
provided that 
\family typewriter
n_stages
\family default
 is 1 or larger.
 
\end_layout

\begin_layout Standard
There can be any number of subscripts
\begin_inset LatexCommand index
name "subscripts"

\end_inset

 for an array.
 We point out, however, that in virtually every application of arrays requiring
 more than two subscripts, there is usually a some underlying concept that
 is much better modeled as an object than as part of a deeply subscripted
\begin_inset LatexCommand index
name "subscripted, deeply"

\end_inset


\begin_inset LatexCommand index
name "deeply subscripted"

\end_inset

 array.
 In the following jagged array example, there are really the concepts of
 unit operation and stream that would be better understood if made explicit.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Arrays\InsetSpace ~
can\InsetSpace ~
be\InsetSpace ~
jagged
\bar default

\begin_inset LatexCommand label
name "lyx:Arrays-can-be"

\end_inset

 Arrays can be sparse
\begin_inset LatexCommand index
name "sparse"

\end_inset

 or jagged
\begin_inset LatexCommand index
name "jagged"

\end_inset

.
 For example:
\end_layout

\begin_deeper
\begin_layout LyX-Code
process[1..3] IS_A set OF integer;
\end_layout

\begin_layout LyX-Code
process[1] :== [2];
\end_layout

\begin_layout LyX-Code
process[2] :== [7,5,3];
\end_layout

\begin_layout LyX-Code
process[3] :== [4,6];
\end_layout

\begin_layout LyX-Code
FOR i in [1..3] CREATE
\end_layout

\begin_layout LyX-Code
   FOR j IN process[i] CREATE
\end_layout

\begin_layout LyX-Code
      flow[i][j] IS_A mass;
\end_layout

\begin_layout LyX-Code
   END FOR;
\end_layout

\begin_layout LyX-Code
END FOR;
\end_layout

\begin_layout Standard
process is an array of sets (not to be confused with a set of sets which
 ASCEND does not have) and flow is an array with six elements spread over
 three rows:
\end_layout

\begin_layout LyX-Code
flow[1][2]
\end_layout

\begin_layout LyX-Code
flow[2][7], flow[2][3], flow[2][5]
\end_layout

\begin_layout LyX-Code
flow[3][4], flow[3][6]
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Arrays\InsetSpace ~
are\InsetSpace ~
also\InsetSpace ~
instances 
\end_layout

\begin_deeper
\begin_layout Standard
Each array is itself an object.
 That is, when you write 
\family typewriter
a[1..2] IS_A real
\family default
, three objects get created: 
\family typewriter
a[1]
\family default
, 
\family typewriter
a[2]
\family default
, and 
\family typewriter
a
\family default
.
 The object 
\family typewriter
a
\family default
 is an array instance which has parts named 
\family typewriter
[1]
\family default
 and 
\family typewriter
[2]
\family default
 that are real instances.
 When a parameterized model requires an array, you pass it the single item
 
\family typewriter
a
\family default
, not the elements 
\family typewriter
a[1..2]
\family default
.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
No\InsetSpace ~
contiguous\InsetSpace ~
storage
\begin_inset LatexCommand index
name "contiguous storage, no"

\end_inset

 
\end_layout

\begin_deeper
\begin_layout Standard
Just in case you still have not caught on, ASCEND arrays are not blocks
 of memory such as are seen in low-level languages like C
\begin_inset LatexCommand index
name "C, computer language"

\end_inset

, FORTRAN
\begin_inset LatexCommand index
name "FORTRAN"

\end_inset

, and Matlab
\begin_inset LatexCommand index
name "Matlab"

\end_inset

.
 The modeling language does not provide things like MatMult
\begin_inset LatexCommand index
name "MatMult"

\end_inset

, Transpose
\begin_inset LatexCommand index
name "Transpose"

\end_inset

, and Inverse
\begin_inset LatexCommand index
name "Inverse"

\end_inset

 because these are procedural solving tools.
 If you are dedicated, you could write METHODs that implement matrix algebra,
 but this is a really dumb idea.
 We aim to structure our software so that it can interact openly with separate,
 dedicated tools (such as Matlab) when those tools are needed.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Index\InsetSpace ~
variable
\begin_inset LatexCommand index
name "index variable"

\end_inset

 One can introduce a variable as an index ranging over a set.
 Index variables are local to the statements in which they occur.
 An example of using an index variable is the following FOR statement:
\end_layout

\begin_deeper
\begin_layout LyX-Code
FOR i IN components CREATE
\end_layout

\begin_layout LyX-Code
   VLE_equil[i]: y[i] = K[i]*x[i];
\end_layout

\begin_layout LyX-Code
END FOR;
\end_layout

\begin_layout Standard
In this example i implicitly is of the same type as the values in the set
 components.
 If another object i exists in the model containing the FOR loop, it is
 ignored while executing the statements in that loop.
 This may cause unexpected results and the compiler will generate warnings
 about loop index shadowed variables.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Label
\begin_inset LatexCommand index
name "label"

\end_inset

: One can label statements which define arithmetic relationships (objective
 functions, equalities, and inequalities) in ASCEND.
 Labeling is highly recommended because it makes models much more readable
 and more easily debugged.
 Labels are also necessary for relations which are going to be used in condition
al modeling or differentiation functions.
 A label is a sequence of alphanumeric characters ending in a colon.
 An example of a labeled equation is:
\end_layout

\begin_deeper
\begin_layout LyX-Code
mass_balance: m_in = m_out;
\end_layout

\begin_layout Standard
An example of a labeled objective function is:
\end_layout

\begin_layout LyX-Code
obj1: MAXIMIZE revenue - cost;
\end_layout

\begin_layout Standard
If a relation is defined within a FOR statement, it must have an array indexed
 label so that each instance created using the statement is distinguishable
 from the others.
 An example is:
\end_layout

\begin_layout LyX-Code
FOR i IN components CREATE
\end_layout

\begin_layout LyX-Code
   equil[i]: y[i] = K[i]*x[i];
\end_layout

\begin_layout LyX-Code
END FOR;
\end_layout

\begin_layout Standard
The ASCEND interactive user interface identifies relationships by their
 labels.
 If one has not provided such a label, the system generates the label:
\end_layout

\begin_layout LyX-Code

\emph on
modelname_equationnumber
\end_layout

\begin_layout Standard
where modelname and equationnumber are the name of the model and the equation
 number in the model.
 An example is
\end_layout

\begin_layout LyX-Code
mixture_14
\end_layout

\begin_layout Standard
for the unlabeled 14th relation in the mixture definition.
 If there is a conflict caused with an existing name, the generated name
 has enough letters added after equationnumber to make it a unique name.
 Remember that each model in a refinement hierarchy inherits the equations
 of its less refined ancestors, so the first equation appearing in the source
 code of a refining model may actually be the nth relation in that model.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Lists
\bar default

\begin_inset LatexCommand index
name "lists"

\end_inset

 Often in a statement one can include a list of names or expression.
 A name list is one or more names where multiple list entries are separated
 from each other by commas.
 Examples of a list of names are:
\end_layout

\begin_deeper
\begin_layout LyX-Code
\align block
T1, inlet_T, outlet_T
\end_layout

\begin_layout LyX-Code
\align block
y[components], y_in
\end_layout

\begin_layout LyX-Code
\align block
stage[1..n_stages]
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Ordered\InsetSpace ~
lists
\begin_inset LatexCommand index
name "lists, ordered"

\end_inset


\begin_inset LatexCommand index
name "ordered lists"

\end_inset

:
\bar default
 If the ordering of names in a list matters, that list is enclosed in ().
 Order matters in: calling externally defined methods or models, calling
 most real-valued functions, passing parameters to ASCEND models or methods,
 and declaring the controlling parameters that SELECT, SWITCH, and WHEN
 statements make decisions on.
\end_layout

\begin_deeper
\begin_layout Section
Basic Concepts
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Instances
\begin_inset LatexCommand index
name "instances"

\end_inset

\InsetSpace ~
and\InsetSpace ~
types
\bar default

\begin_inset LatexCommand index
name "types"

\end_inset

 This is an opportune time to emphasize the distinction between the terms
 instance and type.
 A type in ASCEND is what we define when we declare an ASCEND model or atom.
 It is the formal definition of the attributes (parts) and attribute default
 values that an object will have if it is created using the type definition.
 Methods are associated with types.
\end_layout

\begin_deeper
\begin_layout Standard
In ASCEND there are two meanings (closely related) of an instance.
 
\end_layout

\begin_layout Itemize
An instance is a named part that exists within a type definition.
 
\end_layout

\begin_layout Itemize
An instance is a compiled object.
 
\end_layout

\begin_layout Standard
If one is in the context of the ASCEND interface, the system compiles an
 instance of a model type to create an object with which one carries out
 computations.
 The system requires the user to give a simple name for this simulation
 instance.
 This name given is then the first part of the qualified name for all the
 parts of the compiled object.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Implicit\InsetSpace ~
types
\bar default

\begin_inset LatexCommand index
name "types, implicit"

\end_inset


\begin_inset LatexCommand index
name "types, implicit"

\end_inset

 It is possible to create an instance that does not have a corresponding
 type definition in the library.
 The type of such an instance is said to be 
\series bold
implicit
\series default

\begin_inset Foot
status collapsed

\begin_layout Standard
 (Some people use the word 
\emph on
anonymous
\emph default

\begin_inset LatexCommand index
name "type, anonymous"

\end_inset


\begin_inset LatexCommand index
name "anonymous type"

\end_inset

.
 However, no computable type is anonymous and the implicit type of an instance
 is theoretically computable)
\end_layout

\end_inset

.
 The simplest example of an implicit type is the type of an instance compiled
 from the built-in definition 
\family sans
integer_constant
\family default
.
 For example:
\end_layout

\begin_deeper
\begin_layout LyX-Code
i, j IS_A integer_constant;
\end_layout

\begin_layout LyX-Code
i:== 2;
\end_layout

\begin_layout LyX-Code
j:== 3;
\end_layout

\begin_layout Standard
Instances 
\family typewriter
i
\family default
 and 
\family typewriter
j
\family default
, though of the same formal type, are implicit type incompatible because
 they have been assigned distinct values.
\end_layout

\begin_layout Standard
Instances which are either formally or implicitly type incompatible cannot
 be merged.
 This will be discussed further in Section 
\begin_inset LatexCommand vref
reference "sec:x.3Declarative-statements"

\end_inset

.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Parsing
\begin_inset LatexCommand index
name "parsing"

\end_inset

 Most errors in the declaration of an ASCEND model can be caught at parse
 time because the object type of any well-formed name in an ASCEND definition
 can be resolved or proved ambiguous.
 We cannot prove at parse time whether a specific array element will exist,
 but we can know that should such an element exist, it must be of the type
 with which the array is defined.
 
\end_layout

\begin_deeper
\begin_layout Standard
Ambiguity is warned about loudly because it is caused by either misspelling
 or poor modeling style.
 The simplest example of ambiguity follows.
\end_layout

\begin_layout Standard
Assume a type, stream, and a refinement of stream, heat_stream, which adds
 the new variable 
\family typewriter
H
\family default
.
 Now, if we write:
\end_layout

\begin_layout LyX-Code
MODEL mixer;
\end_layout

\begin_layout LyX-Code
input[1..2] IS_A stream;
\end_layout

\begin_layout LyX-Code
output IS_A heat_stream;
\end_layout

\begin_layout LyX-Code
input[1].H + input[2].H = output.H;
\end_layout

\begin_layout LyX-Code
END mixer;
\end_layout

\begin_layout Standard
We see the parser can find the definition of H in the type heat_stream,
 so 
\family typewriter
output.H
\family default
 is well defined.
 The author of the mixer model may intend to refine 
\family typewriter
input[1]
\family default
 and 
\family typewriter
input[2]
\family default
 to be objects of different types, say steam_stream and electric_stream,
 where each defines an 
\family typewriter
H
\family default
 suitable for use in the equation.
 The parser cannot read the authors mind, so it warns that 
\family typewriter
input[1].H
\family default
 and 
\family typewriter
input[2].H
\family default
 are ambiguous in the mixer definition.
 The mixer model is not highly reusable except by the author, but sometimes
 reusability is not a high priority objective.
 The mixer definition is allowed, but it may cause problems in instantiation
 if the author has forgotten the assumption that is not explicitly stated
 in the model and neglects to refine the input streams appropriately.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Instantiation
\bar default

\begin_inset LatexCommand index
name "instantiation"

\end_inset

 Creating an simulation based on a type definition is a multi-phase
\begin_inset LatexCommand index
name "multi-phase"

\end_inset

 process called compiling
\begin_inset LatexCommand index
name "compiling"

\end_inset

 (or instantiation).
 When an instantiation cannot be completed because some structural parameter
 (a symbol_constant, real_constant, boolean_constant, integer_constant,
 or set) does not have a value there will be PENDING
\begin_inset LatexCommand index
name "PENDING"

\end_inset

 statements.
 The user interface will warn that something is incomplete.
\end_layout

\begin_deeper
\begin_layout Standard
In phase
\begin_inset LatexCommand index
name "phases, compiler"

\end_inset

 1 all statements that create instance structures or assign constant values
 are executed.
 This phase theoretically requires an infinite number of passes through
 the structural statements of a definition.
 We allow a maximum of 5 and have never needed more than 3.
 There may be pending statements at the end of phase 1.
 The compiler or interface will issue warnings about pending statements,
 starting with warnings about unassigned constants.
\end_layout

\begin_layout Standard
Phase 2 compiles as many real arithmetic relation definitions as possible.
 Some relations may be impossible to compile because the constants or sets
 they depend on do not have values assigned.
 Other relations may be impossible because they reference variables that
 do not exist.
 This is determined in a single pass.
\end_layout

\begin_layout Standard
Phase 3 compiles as many logical arithmetic relation definitions as possible.
 Some relations may be impossible to compile because the constants or sets
 they depend on do not have values assigned.
 Other relations may be impossible because they reference real arithmetic
 relations that do not exist.
 This is determined in a single pass.
\end_layout

\begin_layout Standard
Phase 4 compiles as many conditional programming statements (
\family sans
WHEN
\family default
s) as possible.
 Some 
\family sans
WHEN
\family default
 relations may be impossible to compile because the discrete variables,
 models, or relations they depend on do not exist.
 This is determined in a single pass.
\end_layout

\begin_layout Standard
Phase 5 executes the variable defaulting statements made in the declarative
 section of each model 
\emph on
if and only if
\emph default
 there are no pending statements from phases 1-4 anywhere in the simulation.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
default_self
\begin_inset LatexCommand index
name "default\\_self"

\end_inset

 After all phases are done, the method default_self is called in the top-most
 model of the simulation, if this method exists.
\end_layout

\begin_deeper
\begin_layout Standard
The first occurrence of each impossible statement will be explained during
 a failed compilation.
 Impossible statements include:
\end_layout

\begin_layout Itemize
Relations containing undefinable variables (often misspellings).
\end_layout

\begin_layout Itemize
Assignments that are dimensionally inconsistent or containing mismatched
 types.
\end_layout

\begin_layout Itemize
Structure building or modifying statements that refer to model parts which
 cannot exist or that require a type-incompatible argument, refinement,
 or merge.
\end_layout

\begin_layout Chapter
Data Type Declarations
\begin_inset LatexCommand index
name "data type declarations"

\end_inset


\end_layout

\begin_layout Standard
In the spectrum of OO languages, ASCEND is best considered as being class-based
\begin_inset LatexCommand index
name "class-based"

\end_inset

, though it is rather more a hybrid.
 We have atom and model definitions, called 
\series bold
types
\series default
, and the compiled objects themselves, called 
\series bold
instances
\series default
.
 ASCEND instances have a record of what type they were constructed from.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Type\InsetSpace ~
qualifiers
\begin_inset LatexCommand index
name "type qualifiers"

\end_inset

:
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
UNIVERSAL
\family default

\begin_inset LatexCommand index
name "UNIVERSAL"

\end_inset

 Universal is an optional modifier of all 
\family sans
ATOM
\family default
, 
\family sans
CONSTANT
\family default
 and 
\family sans
MODEL
\family default
 definitions.
 If 
\family sans
UNIVERSAL
\family default
 precedes the definition, then 
\emph on
all
\emph default
 instances of that type will actually refer to the first instance of the
 type that is created.
 This saves memory and ensures global consistency of data.
 
\end_layout

\begin_deeper
\begin_layout Standard
Examples of universal type definitions are
\end_layout

\begin_layout LyX-Code
UNIVERSAL MODEL methane
\end_layout

\begin_layout LyX-Code
   REFINES generic_component_model;
\end_layout

\begin_layout LyX-Code
UNIVERSAL CONSTANT circle_constant
\end_layout

\begin_layout LyX-Code
   REFINES real_constant :== 1{PI};
\end_layout

\begin_layout LyX-Code
UNIVERSAL ATOM counter_1
\end_layout

\begin_layout LyX-Code
   REFINES integer;
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Tip:\InsetSpace ~
Do\InsetSpace ~
not\InsetSpace ~
use\InsetSpace ~

\family sans
UNIVERSAL
\family default
\InsetSpace ~
variables\InsetSpace ~
in\InsetSpace ~
relations.
\end_layout

\begin_deeper
\begin_layout Standard
It is important to note that, because 
\emph on
variables
\emph default
 must store information about which relations they occur in, it is a very
 bad idea to use 
\family sans
UNIVERSAL
\family default
 typed variables in relations.
 The construction and maintenance of the relation list becomes very expensive
 for universal variables.
 
\family sans
UNIVERSAL
\family default
 
\emph on
constants
\emph default
 are fine to use, though, because there are no relation links for constants.
\end_layout

\begin_layout Section
Models
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
MODEL
\family default

\begin_inset LatexCommand index
name "MODEL"

\end_inset

 An ASCEND model has a declarative part and an optional procedural part
 headed by the METHODS word.
 Models are essentially containers for variables and relations.
 We will explain the various statements that can be made within models in
 Section 
\begin_inset LatexCommand vref
reference "sec:x.3Declarative-statements"

\end_inset

 and Section 
\begin_inset LatexCommand vref
reference "sec:x.4Procedural-statements"

\end_inset

.
\end_layout

\begin_layout Standard
Simple\InsetSpace ~
models
\begin_inset LatexCommand index
name "models, simple"

\end_inset

:
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
foo 
\family typewriter
\InsetSpace ~
\InsetSpace ~
\InsetSpace ~
\InsetSpace ~
MODEL foo;
\end_layout

\begin_deeper
\begin_layout LyX-Code
(* statements about foo go here*)
\end_layout

\begin_layout LyX-Code
METHODS
\end_layout

\begin_layout LyX-Code
(* METHODs for foo go here*)
\end_layout

\begin_layout LyX-Code
END foo;
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
bar 
\family typewriter
\InsetSpace ~
\InsetSpace ~
\InsetSpace ~
\InsetSpace ~
MODEL bar REFINES foo;
\end_layout

\begin_deeper
\begin_layout LyX-Code
(*additional statements about foo *)
\end_layout

\begin_layout LyX-Code
METHODS
\end_layout

\begin_layout LyX-Code
(* additional METHODs for bar *)
\end_layout

\begin_layout LyX-Code
END bar;
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Parameterized\InsetSpace ~
Models
\bar default

\begin_inset LatexCommand index
name "models, parameterized"

\end_inset

 Parameterizing models makes them easier to understand and faster for the
 system to compile.
 The syntax for a parameterized model vaguely resembles a function call
 in imperative languages, but it is NOT.
 When constructing a reusable model, all the constants that determine the
 sizes of arrays and other structures should be declared in the parameter
 list so that 
\end_layout

\begin_deeper
\begin_layout Itemize
the user knows what is required to reuse the model.
\end_layout

\begin_layout Itemize
the compiler knows what values must be set before it should bother attempting
 to compile the model.
\end_layout

\begin_layout Standard
There is no reason that other items could not also go in the parameter list,
 such as key variables which might be considered inputs or outputs or control
 parameters in the mathematical application of the model.
 A simple example of parameterization would be:
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
column(n,s) 
\end_layout

\begin_deeper
\begin_layout LyX-Code
MODEL column(
\end_layout

\begin_layout LyX-Code
   ntrays WILL_BE integer_constant; 
\end_layout

\begin_layout LyX-Code
   components IS_A set of symbol_constant;
\end_layout

\begin_layout LyX-Code
);
\end_layout

\begin_layout LyX-Code
stage[1..ntrays] IS_A simple_tray;
\end_layout

\begin_layout LyX-Code
END column;
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
flowsheet
\end_layout

\begin_deeper
\begin_layout LyX-Code
MODEL flowsheet;
\end_layout

\begin_layout LyX-Code
   tower4size IS_A integer_constant;
\end_layout

\begin_layout LyX-Code
   tower4size :== 22;
\end_layout

\begin_layout LyX-Code
   ct IS_A column(tower4size,[c5,c6]);
\end_layout

\begin_layout LyX-Code
   (* additional flowsheet statements *)
\end_layout

\begin_layout LyX-Code
END flowsheet;
\end_layout

\begin_layout Standard
In this example, the column model takes the first argument, ntrays, by reference.
 That is, 
\family typewriter
ct.ntrays
\family default
 is an alias for the flowsheet instance 
\family typewriter
tower4size
\family default
.
 
\family typewriter
tower4size
\family default
 must be compiled and assigned a value before we will attempt to compile
 the column model instance 
\family typewriter
ct
\family default
.
 The second argument is taken by value, 
\family typewriter
[c5,c6]
\family default
, and assigned to components, a column part that was declared with 
\family sans
IS_A
\family default
 in the parameter list.
 There is only one name for this set, 
\family typewriter
ct.components
\family default
.
 Note that in the 
\family typewriter
flowsheet
\family default
 model there is no part that is a set of 
\family sans
symbol_constant
\family default
.
 
\end_layout

\begin_layout Standard
The use of parameters in ASCEND modeling requires some thought, and we will
 present that set of thoughts in Section 
\begin_inset LatexCommand vref
reference "sec:x.5Parameterized-models"

\end_inset

.
 Beginners may wish to create new models without parameters until they are
 comfortable using the existing parameterized library definitions.
 Parameters are intended to support model reuse and efficient compilation
 which are not issues in the very earliest phase of developing novel models.
\end_layout

\begin_layout Section
Sets
\begin_inset LatexCommand index
name "sets"

\end_inset


\begin_inset LatexCommand label
name "sub:x.2.2Sets"

\end_inset


\end_layout

\begin_layout Standard
Arrays in ASCEND, as already discussed in Section 
\begin_inset LatexCommand vref
reference "sub:x.1.2Basic-Elements"

\end_inset

, are defined over sets.
 A set is simply an instance with a set value.
 The elements of sets are 
\emph on
not
\emph default
 instances or sets.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Set\InsetSpace ~
Declaration:
\bar default
 A set is made of either symbol_constants or integer_constants, so a set
 object is declared in one of two ways:
\end_layout

\begin_deeper
\begin_layout LyX-Code
my_integer_set IS_A set OF integer_constant;
\end_layout

\begin_layout Standard
or
\end_layout

\begin_layout LyX-Code
my_symbol_set IS_A set OF symbol_constant;
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
:==
\family default

\begin_inset LatexCommand index
name ":=="

\end_inset

 A set is assigned a value like so:
\end_layout

\begin_deeper
\begin_layout LyX-Code
my_integer_set :== [1,4];
\end_layout

\begin_layout Standard
The RHS of such an assignment must be either the name of another set instance
 or an expression enclosed in square brackets and made up of only set operators,
 other sets, and the names of integer_constants or symbol_constants.
 Sets can only be assigned once.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Set\InsetSpace ~
Operations
\bar default
 
\family sans
UNION
\family default

\begin_inset LatexCommand index
name "UNION"

\end_inset


\family typewriter
[setlist]
\end_layout

\begin_deeper
\begin_layout Standard
A function taken over a list of sets.
 The result is the set that includes all the members of all the sets in
 the list.
 Note that the result of the 
\family sans
UNION
\family default
 operation is an unordered set and the argument order to the union function
 does not matter.
 The syntax is:
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
+
\begin_inset LatexCommand index
name "+, sets"

\end_inset

 
\family typewriter
UNION[list_of_sets]
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
A+B
\family default
\InsetSpace ~
is\InsetSpace ~
shorthand\InsetSpace ~
for 
\family typewriter
UNION[A,B]
\end_layout

\begin_deeper
\begin_layout Standard
Consider the following sets for the examples to follow.
\end_layout

\begin_layout LyX-Code
A := [1, 2, 3, 5, 9];
\end_layout

\begin_layout LyX-Code
B := [2, 4, 6, 8];
\end_layout

\begin_layout Standard
Then 
\family typewriter
UNION[A, B]
\family default
 is equal to the set 
\family typewriter
[1, 2, 3, 4, 5, 6, 8, 9]
\family default
 which equals 
\family typewriter
[1..6, 8, 9]
\family default
 which equals 
\family typewriter
[[1..9] - [7]]
\family default
.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
INTERSECTION
\family default

\begin_inset LatexCommand index
name "INTERSECTION"

\end_inset

[] INTERSECTION[list of set expressions].
 Finds the intersection (and) of the sets listed.
 
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
*
\begin_inset LatexCommand index
name "*, sets"

\end_inset

 Equivalent to 
\family sans
INTERSECTION
\family typewriter
[list_of_sets]
\family default
.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family typewriter
A*B
\family default
\InsetSpace ~
is\InsetSpace ~
shorthand\InsetSpace ~
for\InsetSpace ~
I
\family typewriter
NTERSECTION[A,B]
\end_layout

\begin_deeper
\begin_layout Standard
For the sets A and B defined just above, 
\family typewriter
INTERSECTION[A, B]
\family default
 is the set 
\family typewriter
[2]
\family default
.
 The 
\family typewriter
*
\family default
 shorthand for intersection is 
\emph on
not
\emph default
 recommended for use except in libraries no one will look at.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000
Set\InsetSpace ~
difference
\begin_inset LatexCommand index
name "set difference"

\end_inset

: One can subtract one set from another.
 The result is the first set less any members in the set union of the first
 and second set.
 The syntax is
\end_layout

\begin_deeper
\begin_layout LyX-Code

\family typewriter
first_set
\family default
 -
\begin_inset LatexCommand index
name "-, sets"

\end_inset

 
\family typewriter
second_set
\end_layout

\begin_layout Standard
For the sets 
\family typewriter
A
\family default
 and 
\family typewriter
B
\family default
 defined above, the set difference 
\family typewriter
A - B
\family default
 is the set 
\family typewriter
[1, 3, 5, 9]
\family default
 while the set difference 
\family typewriter
B - A
\family default
 is the set 
\family typewriter
[4, 6, 8]
\family default
.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
CARD
\family default

\begin_inset LatexCommand index
name "CARD"

\end_inset

[set] Cardinality
\begin_inset LatexCommand index
name "cardinality"

\end_inset

.
 Returns an integer constant value that is the number of items in the set.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
CHOICE
\family default

\begin_inset LatexCommand index
name "CHOICE"

\end_inset

[set] Choose one.
 The result of running the 
\family sans
CHOICE
\family default
 function over a set is an arbitrary (but consistent: for any set instance
 you always get the same result) single element of that set.
 
\end_layout

\begin_deeper
\begin_layout Standard
Running 
\family typewriter
CHOICE[A]
\family default
 gives any member from the set A.
 The result is a member, not a set.
 To make the result into a set, it must be enclosed in square brackets.
 Thus 
\family typewriter
[CHOICE[A]]
\family default
 is a set with a single element arbitrarily chosen from the set A.
 Good modelers do not leave modeling decisions to the compiler; they do
 not use 
\family sans
CHOICE
\family default
.
 We are stuck with it for historical reasons.
\end_layout

\begin_layout Standard
To reduce a set by one element, one can use the following
\end_layout

\begin_layout LyX-Code
A_less_one IS_A set OF integer;
\end_layout

\begin_layout LyX-Code
A_less_one :== A - [CHOICE[A]];
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
IN
\family default

\begin_inset LatexCommand index
name "IN"

\end_inset

 
\family typewriter
lhs IN rhs
\family default
 can only be well explained by examples.
 
\family sans
IN
\family default
 is used in index expressions.
 If 
\family typewriter
lhs
\family default
 is a simple and not previously defined name, it is created as a temporary
 loop index which will take on the values of the rhs set definition.
 If lhs is something that already exists, the result of 
\family typewriter
lhs IN rhs
\family default
 is a boolean value; stare at the model 
\family typewriter
set_example
\family default
 below which demonstrates both 
\family sans
IN
\family default
 and 
\family sans
SUCH_THAT
\family default
.
 If you still are not satisfied, you might examine [[westerbergksets]]
\begin_inset Note Note
status open

\begin_layout Standard
fix this reference
\end_layout

\end_inset

.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
SUCH_THAT
\family default

\begin_inset LatexCommand index
name "SUCH\\_THAT"

\end_inset

 Set expressions can be rather clever.
 We will give a detailed example from chemistry because unordered sets are
 unfamiliar to most people and set arithmetic is quite powerful.
 In this example we see arrays of sets and sparse arrays.
 
\end_layout

\begin_layout LyX-Code
MODEL set_example;
\end_layout

\begin_layout LyX-Code
  (* we define a sparse matrix of reaction
\end_layout

\begin_layout LyX-Code
     coefficient information and the species
\end_layout

\begin_layout LyX-Code
     balance equations.
 *) 
\end_layout

\begin_layout LyX-Code
  rxns IS_A set OF integer_constant;
\end_layout

\begin_layout LyX-Code
  rxns :== [1..3];
\end_layout

\begin_layout LyX-Code
  species IS_A set OF symbol_constant;
\end_layout

\begin_layout LyX-Code
  species :== ['A','B','C','D'];
\end_layout

\begin_layout LyX-Code
  reactants[rxns] IS_A set OF symbol_constant; (* species
\end_layout

\begin_layout LyX-Code
     in each rxn_j *)
\end_layout

\begin_layout LyX-Code
  reactants[1] :== ['A','B','C'];
\end_layout

\begin_layout LyX-Code
  reactants[2] :== ['A','C'];
\end_layout

\begin_layout LyX-Code
  reactants[3] :== ['A','B','D'];
\end_layout

\begin_layout LyX-Code
  reactions[species] IS_A set OF integer_constant; 
\end_layout

\begin_layout LyX-Code
  FOR i IN species CREATE (* rxns for each species i *)
\end_layout

\begin_layout LyX-Code
    reactions[i] :== [j IN rxns SUCH_THAT i IN reactants[j]];
\end_layout

\begin_layout LyX-Code
  END FOR;
\end_layout

\begin_layout LyX-Code
  (* Define sparse stoichiometric matrix.
 Values of eta_ij
\end_layout

\begin_layout LyX-Code
     set later.*)
\end_layout

\begin_layout LyX-Code
  FOR j IN rxns CREATE
\end_layout

\begin_layout LyX-Code
    FOR i IN reactants[j] CREATE
\end_layout

\begin_layout LyX-Code
      (* eta_ij --> mole i/mole rxn j*)
\end_layout

\begin_layout LyX-Code
      eta[i][j] IS_A real_constant; 
\end_layout

\begin_layout LyX-Code
    END FOR;
\end_layout

\begin_layout LyX-Code
  END FOR;
\end_layout

\begin_layout LyX-Code
  production[species] IS_A molar_rate;
\end_layout

\begin_layout LyX-Code
  rate[rxns] IS_A molar_rate; (* mole rxn j/time *)
\end_layout

\begin_layout LyX-Code
  FOR i IN species CREATE
\end_layout

\begin_layout LyX-Code
    gen_eqn[i]: production[i] = 
\end_layout

\begin_layout LyX-Code
    SUM[eta[i][j]*rate[j] | j IN reactions[i]];
\end_layout

\begin_layout LyX-Code
  END FOR;
\end_layout

\begin_layout LyX-Code
END set_example;
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
"
\family typewriter
|
\family default

\begin_inset LatexCommand index
name "|"

\end_inset

" is shorthand for 
\family sans
SUCH_THAT
\family default
.
\end_layout

\begin_deeper
\begin_layout Standard
The array 
\family typewriter
eta
\family default
 has only 8 elements, and we defined those elements in a set for each reaction.
 The equation needs to know about the set of reactions for a species 
\family typewriter
i
\family default
, and that set is calculated automatically in the models first 
\family sans
FOR/CREATE
\family default
 statement.
 
\end_layout

\begin_layout Standard
The 
\family typewriter
|
\family default
 symbol is the ASCEND III notation for 
\family sans
SUCH_THAT
\family default
.
 We noted that "
\family typewriter
|
\family default
" is often read as "for all", which is different in that "for all" makes
 one think of a 
\family sans
FOR
\family default
 loop where the loop index is on the left of an 
\family sans
IN
\family default
 operator.
 For example, the 
\family typewriter
j
\family default
 loop in the 
\family sans
SUM
\family default
 of 
\family typewriter
gen_eqn[i]
\family default
 above.
 
\end_layout

\begin_layout Section
Constants
\begin_inset LatexCommand index
name "constants"

\end_inset


\end_layout

\begin_layout Standard
ASCEND supports real, integer, boolean and character string constants.
 Constants in ASCEND do not have any attributes other than their value.
 Constants are scalar quantities that can be assigned exactly once.
 Constants may only be assigned using the 
\family typewriter
:==
\family default
 operator and the RHS expression they are assigned from must itself be constant.
 Constants do not have subparts.
 Integer and symbol constants may be used in determining the definitions
 of sets.
 
\end_layout

\begin_layout Standard
Explicit refinements of the built-in constant types may be defined as exemplifie
d in the description of real_constant.
 Implicit type refinements may be done by instantiating an incompletely
 defined constant and assigning its final value.
 
\end_layout

\begin_layout Standard
Sets could be considered constant because they are assigned only once, however
 sets are described separately because they are not quite scalar quantities.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
real_constant
\family default

\begin_inset LatexCommand index
name "real\\_constant"

\end_inset

 Real number with dimensionality.
 Note that the dimensionality of a real constant can be specified via the
 type definition without immediately defining the value, as in the following
 pair of definitions.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
CONSTANT\InsetSpace ~
declaration\InsetSpace ~
example:
\end_layout

\begin_deeper
\begin_layout LyX-Code
CONSTANT molar_weight
\end_layout

\begin_layout LyX-Code
   REFINES real_constant DIMENSION M/Q;
\end_layout

\begin_layout LyX-Code
CONSTANT hydrogen_weight
\end_layout

\begin_layout LyX-Code
   REFINES molar_weight :== 1.004{g/mole};
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
integer_constant
\family default

\begin_inset LatexCommand index
name "integer\\_constant"

\end_inset

 Integer number.
 Principally used in determining model structure.
 If appearing in equations, integers are evaluated as dimensionless reals.
 Typical use is inside a 
\family sans
MODEL
\family default
 definition and looks like:
\end_layout

\begin_deeper
\begin_layout LyX-Code
n_trays IS_A integer_constant;
\end_layout

\begin_layout LyX-Code
n_trays :== 50;
\end_layout

\begin_layout LyX-Code
tray[1..n_trays] IS_A vl_equilibrium_tray;
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
symbol_constant
\family default

\begin_inset LatexCommand index
name "symbol\\_constant"

\end_inset

 Object with a symbol value.
 May be used in determining model structure.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
boolean_constant
\family default

\begin_inset LatexCommand index
name "boolean\\_constant"

\end_inset

 Logical value.
 May be used in determining model structure.
\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000

\bar under
Setting\InsetSpace ~
constants
\bar default

\begin_inset LatexCommand index
name "constants, setting"

\end_inset


\end_layout

\begin_layout List
\labelwidthstring 00000.00000.00000.000
:==
\begin_inset LatexCommand index
name ":=="

\end_inset

 Constant and set assignment operator.
 
\end_layout

\begin_deeper
\begin_layout Standard
It is suggested, but not required, that names of all types that refine the
 built-in constant types have names that end in _constant.
\end_layout

\begin_layout LyX-Code
LHS_list :== RHS; 
\end_layout

\begin_layout Standard
Here it is required that the one or more items in the LHS be of the same
 constant type and that RHS is a single-valued expression made up of values,
 operators, and other constants.
 The 
\family typewriter
:==
\family default
 is used to make clear to both the user and the system what scalar objects
 are constants.
 
\end_layout

\begin_layout Section
Variables
\begin_inset LatexCommand index
name "variables"

\end_inset


\end_layout

\begin_layout Standard
There are four built-in types which may be used to construct variables:
 symbol, boolean, integer, and real.
 At this time symbol types have special restrictions.
 Refinements of these variable base types are defined with the 
\family sans
ATOM
\family default
 statement.
 Atom types may declare attribute fields with types real, integer, boolean,
 symbol, and set.
 These attributes are 
\emph on
not
\emph default
 independent objects and therefore cannot be refined, merged, or put in
 a refinement clique (
\family sans
ARE_ALIKE
\family default
d).
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
ATOM
\family default

\begin_inset LatexCommand index
name "ATOM"

\end_inset

 The syntax for declaring a new atom type is
\end_layout

\begin_deeper
\begin_layout LyX-Code
ATOM 
\emph on
atom_type_name
\emph default
 REFINES 
\emph on
variable_type
\end_layout

\begin_layout LyX-Code
   «DIMENSION 
\emph on
dimension_expression
\emph default
»
\end_layout

\begin_layout LyX-Code
   «DEFAULT 
\emph on
value
\emph default
»; (* note the ; *)
\end_layout

\begin_layout LyX-Code
   «
\emph on
initial attribute assignment
\emph default
;»
\end_layout

\begin_layout LyX-Code
END 
\emph on
atom_type_name
\emph default
;
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
DEFAULT
\family default

\begin_inset LatexCommand index
name "DEFAULT"

\end_inset

,
\family sans
\InsetSpace ~
DIMENSION
\family default

\begin_inset LatexCommand index
name "DIMENSION"

\end_inset

,\InsetSpace ~
and\InsetSpace ~

\family sans
DIMENSIONLESS
\family default

\begin_inset LatexCommand index
name "DIMENSIONLESS"

\end_inset


\end_layout

\begin_deeper
\begin_layout Standard
The 
\family sans
DIMENSION
\family default
 attribute is for variables whose base type is real.
 It is an optional field.
 If not defined for any atom with base type real, the dimensions will be
 left as undefined.
 Any variable which is later declared to be one of these types will be given
 wild card dimensionality (represented in the interactive display by an
 asterisk (*)).
 The system will deduce the dimensionality from its use in the relationships
 in which it appears or in the declaring of default values for it, if possible.
\end_layout

\end_deeper
\begin_layout List
\labelwidthstring 00000.00000.00000.000

\family sans
solver_var
\family default

\begin_inset LatexCommand index
name "solver\\_var"

\end_inset

 is a special case of 
\family sans
ATOM
\family default
 and we will say much more about it in Section 
\begin_inset LatexCommand vref
reference "sub:x.6.1Variables-for-solvers"

\end_inset

.