The last element of a list 
		?- last([1,2,3,4],R).
		R = 4

		last(L,R):- append(X,[R],L).

		The last 2 elements of a list
		?- last2([1,2,3,4],R).
		R = [3,4]
				
		last2(L,[A,B]):- append(X,[A,B],L).

		The last n elements of a list L.
		?-lastn([1,2,3,4],2,T).
		T = [3,4]
		
		lastn(L,N,Y):- append(X,Y,L),length(Y,N).

		The first n elements of a list. 
		?- firstn([1,2,3,4,5],3,W).
		W = [1,2,3]
			
		firstn(L,N,X):- append(X,Y,L),length(X,N).