**Rules with a Guard

The full syntax of a rule is: ( '''Head''' :- ['''Guard''' | ] '''Body''' )

where '''Guard''' is a multiset of ''type constraints'' of the form:

Type constraints put constraints on the shapes of processes
(or the names of unary atoms)
with which the process contexts specified in its arguments can match.
The ''type constraint name'' &math(p); is drawn from a built-in set and specifies which kind of constraints is imposed.


Here is an example rule with guard:

 wait($p) :- int($p) | ok.

This rule can be thought of as an abbreviation
of the following infinite number of rules:

 wait(0) :- ok.
 wait(1) :- ok.
 wait(-1):- ok.
 wait(2) :- ok.

The following list contains examples of some type constraints that can be written in '''Guard''':

- int($p) --- specifies that $p must be an integer atom.
- 4($p) --- specifies that $p must be a unary integer atom of value 4 (i.e., 4(X)).
- $p < $q --- specifies that $p and $q are integer atoms
such that the value of $p is less than that of $q.
- $r = $p +. $q --- specifies that $p, $q, and $r are floating
point number atoms
such that the sum of the values of $p and $q is equal to the value of $r. 


In reality, each type constraint name (such as int or <)
has its own mode of usage
that specifies which of its arguments are input arguments.
The effect of the constraint specified by a type constraint
is enabled only after the shapes (or values)
of its input arguments are all determined.

The same abbreviation scheme as defined for atoms
applies to type constraints when a process context name &math(\$p_i);
occurs exactly two times in the rule.
For example, p($n):-$n>$z,0($z)|ok can be abbreviated
to p($n):-$n>0|ok.

**Typed Process Contexts

A process context name $p constrained in '''Guard'''
is said to be ''typed'' in that rule.
As a syntactic sugar,
typed process context names can be written as link names as follows:

 ( Res = gen(N) :- N > 0 | Res = [N|gen(N-1)] ), p(gen(10))

**Guard Library

Currently, the following type constraints can be written in the
guard. The + specifies an input argument.

 '='(+Unary,-Unary)         - equivalence
 '='(-Unary,+Unary)         - equivalence
 '=='(+Unary,+Unary)        - equivalence
 int(+Float,-Int)           - cast
 float(+Int,-Float)         - cast
 345(-Int)                  - for every integer,  not only with 345
 '-3.14'(-Float)            - for every floating, not only with -3.14
 '<'(+Int,+Int)             - integer comparison,  as well as: > =< >= =:= =\=
 '+'(+Int,+Int,-Int)        - integer operation,   as well as: - * / mod
 '<.'(+Float,+Float)        - floating comparison, as well as: >. =<. >=. =:=. =\=.
 '+.'(+Float,+Float,-Float) - floating operation,  as well as: -. *. /.

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