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//[[Documentation]]

*Hyperlinks (slim)

LMNtal links represent point-to-point connection between atoms and form an ordinary graph.  In some applications, however, we may want '''hyperlinks''', namely links that interconnect more than two atoms and form a '''hypergraph'''.

LMNtal provides syntax and operations for hypergraphs, which will be described below.  Currently, hyperlinks are supported by the slim runtime
in the ordinary (as opposed to non-deterministic) execution mode. 

**Creating a hyperlink

-A hyperlink is created by a "new" construct specified in a guard.

 hoge.
 hoge :- new($h) | a($h), b($h).
 *--> a(!1), b(!1).


**Type checking for hyperlinks
-hlink($x)

 a(!1).
 a($x) :- hlink($x) | b($x).
 *--> b(!1).

**Merging two hyperlinks

-Two hyperlinks can be merged into one using the "><" operator.

 a(!1), b(!2).
 a($x), a($y) :- $x \= $y | a($x), b($y), $x >< $y.
--If $x and $y represent different hyperlinks, they will be merged into one.

***Number of occurrences
-The "num" construct in a guard will return the number of occurrences of the specified hyperlink.

 a(!1), b(!1).
 a($h) :- $n = num($h) | number($n).
 *--> b(!1), number(2).  
--Another use
 a(!1).
 a($h) :- $n = num($h), $n < 5 | a($h), a($h).
 *--> a(!1), a(!1), a(!1), a(!1), a(!1).
--Yet another use
 flag(!1), a(!1), a(!1), a(!1), a(!1).
 a($h) :- hlink($h) | .
 flag($h) :- $n = num($h), $n =:= 1 | ok.
 *--> ok. // ok is created when all a(!1)'s are removed
---Thus, using hyperlinks, one can check the absence of a particular atom.

**Guard Library

The following type constraints can be used in guards.
The + (input) sign preceding a process context name means that the name should
appear in the head, while the - (output) sign means that the name should not
appear in the head.

:'='(+$u,-$v)|make sure that $u[X] and $v[Y] are unary atoms with the same name.
:'='(-$u,+$v)|same as above.
:'=='(+$u,+$v)|check if $u[X] and $v[Y] are unary atoms with the same name.
:unary(+$u)|check if $u[X] is a unary atom.
:ground(+$g)|check if $g[X1,...,Xn] (n>0) is a connected graph whose free links are exactly X1,...,Xn.
:int(+$i)|check if $i[X] is an integer.
:float(+$f)|check if $f[X] is a floating-point number.
:int(+$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,+$int)|integer comparison; also: ''&color(#8B4513){'>'};'', ''&color(#8B4513){'=<'};'', ''&color(#8B4513){'>='};'', ''&color(#8B4513){'=:='};'', ''&color(#8B4513){'=\='};''.
:'+'(+$int,+$int,-$int)|integer operation;  also: ''&color(#8B4513){'-'};'', ''&color(#8B4513){'*'};'', ''&color(#8B4513){'/'};'', ''&color(#8B4513){mod};''.
:'<.'(+$float,+$float)|float comparison;   also: ''&color(#8B4513){'>.'};'', ''&color(#8B4513){'=<.'};'', ''&color(#8B4513){'>=.'};'', ''&color(#8B4513){'=:=.'};'', ''&color(#8B4513){'=\=.'};''.
:'+.'(+$float,+$float,-$float)|float operation;    also: ''&color(#8B4513){'-.'};'', ''&color(#8B4513){'*.'};'', ''&color(#8B4513){'/.'};''.
:uniq(+$g1,...,+$gn)|uniqueness constraint; checks if the rule has not been applied to the tuple $g1[X1], ..., $gn[Xn] (n>=0).

Backup source of Hyperlinks (No. 1)