comb(N,L,Result). Combinations of N elements from the list L. 
		?- comb(2,[a,b,c],I).
		I = [a,b] 
		I = [a,c] 
		I = [b,c] 

		comb(N,L,X):- length(X,N),mem1(X,L).
	rest(A,List,Rest). Returns the Rest of the list after the first occurrence of A. 
		?- rest(a,[a,b,c,d],R).
		R = [b,c,d]
		?- rest(a,[b,c,a,d],I).
		R = [d]
		?- rest(a,[b,c,d],R).
		R = []

		rest(X,[],[]):- !.
		rest(X,[X|T],T):- !.
		rest(X,[_|T],R):- rest(X,T,R).


		mem1(Lr,L). Fills Lr with elements from L without repetitions	
		?- mem1([X,Y],[a,b,c]),write([X,Y]),fail.
		[a,b][a,c][b,a][b,c][c,a][c,b]no

		mem1([],Y).
		mem1([H|T],Y):- member(H,Y),rest(H,Y,New),mem1(T,New).