Appending two lists 'append([a],[b,c,d],Z).' append([a],[b,c,d],[a,b,c,d]) Which lists appends to a known list to form another known list 'append(X,[b,c,d],[a,b,c,d]).' append([a],[b,c,d],[a,b,c,d]) Which lists append to form a known list 'append(X,Y,[a,b,c,d]).' append([],[a,b,c,d],[a,b,c,d]) append([a],[b,c,d],[a,b,c,d]) append([a,b],[c,d],[a,b,c,d]) append([a,b,c],[d],[a,b,c,d]) append([a,b,c,d],[],[a,b,c,d])