A list represents a set if no element repeats.
	 	So set/1 verifies if each element occurs only once in a list.

	  	?- is_set([a,b,c,c,d]).
	    	false.
		?- is_set([a,b,c]).
		true.
		?- is_set([a,[b,a],c]).
		true.
		?- is_set([]).
		true.

	  	myset([]).
	  	myset([X|T]):- not(member(X,T)),myset(T).