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The following is a formal specification of the grammar for ode
's
input language, in BackusNaur form. Nonterminal symbols in the
grammar are enclosed in angle brackets. Terminal tokens are in all
capitals. Bare words and symbols stand for themselves.
<program> ::= ... empty ...
 <program> <statement>
<statement> ::= SEP
 IDENTIFIER = <const> SEP
 IDENTIFIER ' = <expression> SEP
 print <printlist> <optevery> <optfrom> SEP
 step <const> , <const> , <const> SEP
 step <const> , <const> SEP
 examine IDENTIFIER SEP
<printlist> ::= <printitem>
 <printlist> , <printitem>
<printitem> ::= IDENTIFIER
 IDENTIFIER '
 IDENTIFIER ?
 IDENTIFIER !
 IDENTIFIER ~
<optevery> ::= ... empty ...
 every <const>
<optfrom> ::= ... empty ...
 from <const>
<const> ::= <expression>
<expression> ::= ( <expression> )
 <expression> + <expression>
 <expression>  <expression>
 <expression> * <expression>
 <expression> / <expression>
 <expression> ^ <expression>
 FUNCTION ( <expression> )
  <expression>
 NUMBER
 IDENTIFIER
Since this grammar is ambiguous, the following table summarizes the
precedences and associativities of operators within expressions.
Precedences decrease from top to bottom.
Class Operators Associativity
Exponential ^ right
Multiplicative * / left
Additive +  left
As noted in the grammar, there are six types of nontrivial statement.
We now explain the effects (the `semantics') of each type, in turn.

IDENTIFIER ' = <expression>
This defines a firstorder differential equation.
The derivative of IDENTIFIER is specified by <expression>. If a
dynamic variable does not appear on the left side of a statement of this
form, its derivative is assumed to be zero. That is, it is a
symbolic constant.

IDENTIFIER = <const>
This sets the value of IDENTIFIER to the current value of
<expression>. Dynamic variables that have not been initialized in
this way are set to zero.

step <const> , <const>

step <const> , <const> , <const>
A `step' statement causes the numerical scheme to be executed. The
first <const> is the initial value of the independent variable. The
second is its final value. The third is a stepsize; if given, it
overrides any stepsize that may be specified on the command line.
Usually the stepsize is not specified, and it varies adaptively as the
computation proceeds.

print <printlist> [ every <const> ] [ from <const> ]
A `print' statement controls the content and frequency of the
numerical output. <printlist> is a commaseparated list of
IDENTIFIERs, where each IDENTIFIER may be followed by `'',
denoting the derivative, or `?', denoting the relative singlestep
error, or `!', denoting the absolute singlestep error, or
`~', denoting the accumulated error (not currently implemented).
The specified values are printed in the order they are found. Both the
`every' clause and the `from' clause are optional. If the
`every' clause is present, a printing occurs every <const>
iterations of the numerical algorithm. The default is to print on every
iteration (i.e. `every 1'). The first and last values are always
printed. If the `from' clause is present, it means to begin
printing when the independent variable reaches or exceeds <const>.
The default is to begin printing immediately.
If no `print' statement has been supplied, then the independent
variable and all dependent variables which have differential equations
associated with them are printed. The independent variable is printed
first; the dependent variables follow in the order their equations were
given.

examine IDENTIFIER
An `examine' statement, when executed, causes a table of
interesting information about the named variable to be printed on the
standard output. For example, if the statement `examine y' were
encountered after execution of the `ode to Euler' example discussed
elsewhere, the output would be:
"y" is a dynamic variable
value:2.718282
prime:2.718282
sserr:1.121662e09
aberr:3.245638e09
acerr:0
code: push "y"
The phrase `dynamic variable' means that there is a differential
equation describing the behavior of y. The numeric items in the
table are:
 value

Current value of the variable.
 prime

Current derivative of the variable.
 sserr

Relative singlestep error for the last step taken.
 aberr

Absolute singlestep error for the last step taken.
 acerr

Total error accumulated during the most recent `step' statement.
Not currently implemented.
The `code' section of the table lists the stack operations required
to compute the derivative of y (somewhat reminiscent of a
reverse Polish calculator). This information may be useful in
discovering whether the precedences in the differential equation
statement were interpreted correctly, or in determining the time or
space expense of a particular calculation. `push "y"' means to
load y's value on the stack, which is all that is required to
compute its derivative in this case.
The grammar for the ode
input language contains four types of
terminal token: FUNCTION, IDENTIFIER, NUMBER, and
SEP. They have the following meanings.

FUNCTION
One of the words: abs, sqrt, exp, log, ln,
log10, sin, cos, tan, asin, acos, atan,
sinh, cosh, tanh, asinh, acosh, atanh,
floor, ceil, besj0, besj1, besy0, besy1,
erf, erfc, inverf, lgamma, gamma, norm,
invnorm, ibeta, igamma. These are defined to have the same
meaning as in the plotting program
gnuplot
. All functions take a
single argument, except for ibeta, which takes three, and
igamma, which takes two. For trigonometric functions, all arguments
are expressed in radians. The atan function is defined to give a
value between PI/2 and PI/2 (inclusive).

IDENTIFIER
A sequence of alphanumeric characters starting with an alphabetic
character. The first 32 characters are significant. Upper and
lowercase letters are distinct. In identifiers, the underscore
character is considered alphabetic. Function names and keywords may not
be used as identifiers, nor may `PI'.

NUMBER
A nonempty sequence of digits possibly containing a decimal point and
possibly followed by an exponent. An exponent is `e' or
`E', followed by an (optionally signed) one, two, or threedigit
number. All numbers and all parts of numbers are radix 10. A
number may not contain any white space. The special word `PI' is
a number.

SEP
A separator: a semicolon or a (nonescaped) newline.
In the ode
input language, upper and lowercase letters are
distinct. Comments begin with the character `#' and continue to
the end of the line. Long lines may be continued onto a second line by
ending the first line with a backslash (`\'). That is because
the combination backslashnewline is equivalent to a space.
Spaces or tabs are required in the input whenever they are needed to
separate identifiers, numbers, and keywords from one another. Except as
separators, they are ignored.
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