
union_var(?SetVariable, ?UnionVar)

   Union of a set of sets

Arguments
   SetVariable         A Set (variable or ground) of sets.
   UnionVar            A Set (variable or ground).

Type
   library(cardinal)

Description
UnionVar is the union of sets in SetVariable. If UnionVar is given
		(as a ground set or a set variable), then SetVariable is constrained to have such union.
		If UnionVar is a free variable, then it is unified with the set's union as
		a set variable or a ground set (if it is already known).
		union_var/2 can thus be used either to declare (or constrain) a union
		function or to retrieve it.

Fail Conditions
   Fails if UnionVar can not be the union of SetVariable.

Resatisfiable
   No.

Examples
   
?- S `::[]..[[a],[b]], union_var(S,U).
?- S `::[]..[[a],[b],[a,b]], union_var(S,[a,b]).
?- union_var([[a,b],[b,c]], U).
U = [a,b,c]

See Also
   set / 4, sets / 4, cardinality / 2
