**Rules with a Guard

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'''&size(10){1};, ..., $'''p'''&size(10){'''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'''&size(10){1};, ..., $'''p'''&size(10){'''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
occurs exactly twice 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.
For inscance, the above example can be written as:
 waitint(X) :- int(X) | ok.
// ( Res = gen(N) :- N > 0 | Res = [N|gen(N-1)] ), p(gen(10))

**Avoiding Infinite Rule Application

A constraint of the form
''uniq''($'''p'''&size(10){1};, ..., $'''p'''&size(10){'''n'''};)
succeeds if each $'''p'''&size(10){'''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'''&size(10){1};, ..., $'''p'''&size(10){'''n'''};

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.

**Guard Library

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)

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