difflist([H|T],[H|D]-P) :- difflist(T,D-P).  
difflist([],X-X). 

normlist(L-P,L):- P = [].

join(A-B, B-C, A-C).

mergesort([],[]).
mergesort([X],[X]).
mergesort([A,B|Z],R):- 
	split([A,B|Z],X,Y),
	mergesort(X,Xs),
	mergesort(Y,Ys),
	merge(Xs,Ys,R).

split([],[],[]).
split([X],[X],[]).
split([A,B|T],[A|X],[B|Y]) :- split(T,X,Y).  

merge([], [], []).
merge(X,[],X).
merge([],X,X).
merge([H1|X], [H2|Y], R) :- 
	H1 < H2, R=[H1|R1], merge(X, [H2|Y], R1);
	H1 >= H2, R=[H2|R1], merge([H1|X], Y, R1).