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subset(?SubList, +List)
Succeeds if List is the list which contains all elements from SubList in
the same order as in SubList.
- ?SubList
- A term which unifies with a list.
- +List
- A term which unifies with a list.
Description
   Used to test if a specified list contains all elements of another list,
   or to generate all sublists of a given list.
   The definition of this Prolog library predicate is:
        subset([],[]).
        subset([X|L],[X|S]) :-
            subset(L,S).
        subset(L, [_|S]) :-
            subset(L,S).
   This predicate does not perform any type testing functions.
   This predicate works properly for set operations only, so repeated
   elements, variable elements and unsorted lists should not be used.
	
Modes and Determinism
Fail Conditions
   Fails if SubList does not unify with a list whose elements are all
   contained in List in the same order as in SubList.
Resatisfiable
   Yes.
Examples
Success:
      subset([1,3], [1,2,3]).
      subset(X, [1,3,4]).        % backtracks over all subsets
Fail:
      subset([2,1], [1,2,3]).   % different order
See Also
union / 3, subtract / 3, intersection / 3