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The syntax of a rule with a guard is:

Head:-Guard|Body

where *Guard* is a multiset of **type constraints** of the form:
*p*($*p*1, ..., $*p**n*).

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** *p* is drawn from a built-in set and specifies which kind of constraints is imposed.

A constraint of the form **uniq**($*p*1, ..., $*p**n*) is
also allowed. This is a control structure rather than a type constraint
and used to avoid infinite rule application (see below).

Here is an example rule with guard:

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

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

waitint(0) :- ok. waitint(1) :- ok. waitint(-1):- ok. waitint(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.

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. For example, $r = $p + $q proceeds only when $p and $q are determined.

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

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.
For inscance, the above example can be written as:

waitint(X) :- int(X) | ok.

A constraint of the form
**uniq**($*p*1, ..., $*p**n*)
succeeds if each $*p**k* is a *ground* structure
(connected graph with exactly one free link; see below)
and the rule has not
been applied to the tuple
$*p*1, ..., $*p**n*
before.

As a special case of *n*=0,
**uniq** succeeds if the rule in question has not been used
before. The **uniq**() test is a general tool for avoiding
infinite application of rules whose right-hand side is a
super(multi)set of the left-hand side.

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

'='(+U1,-U2) - make sure that U1 and U2 are (connected to) unary atoms with the same name '='(-U1,+U2) - same as above '=='(+U1,+U2) - check if U1 and U2 are (connected to) unary atoms with the same name unary(+U) - check if U is (connected to) a unary atom ground(+G) - check if G is (connected to) a connected graph with exactly one free link (which is G) int(+I) - check if I is (connected to) an integer float(+F) - check if F is (connected to) a float int(+Float,-Int) - cast float(+Int,-Float) - cast 345(-Int) - defined for every integer (not only with 345) '-3.14'(-Float) - defined for every float '<'(+Int,+Int) - integer comparison; also: > =< >= =:= =\= '+'(+Int,+Int,-Int) - integer operation; also: - * / mod '<.'(+Float,+Float) - float comparison; also: >. =<. >=. =:=. =\=. '+.'(+Float,+Float,-Float) - float operation; also: -. *. /. uniq(+G1,...,+Gn) - uniqueness constraint; checks if the rule has not been applied to the tuple G1,...,Gn (n>=0)