```[[Documentation]]

*Guards

**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:
&math(p(\\$p_1,\ldots,\\$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'' &math(p); is drawn from a built-in set and specifies which kind of constraints is imposed.

***Examples

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.

***Notes

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 &math(\\$p_i);
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))

**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: -. *. /.
```