append([],X,X).
append([X|Y], Z, [X|W]) :- append(Y,Z,W).

try(done) :- append([1,2,3],[1,2],X) , append([1],[2,3],Y), Z is 3+4, W is 12-5.

?- X = 3, Y = 5, X =< Y, write(yes).
