You know, if you keep learning you will never cease to have this feeling.

  ?- append([1, 2, 3], [4, 5, 6], X).
  X = [1, 2, 3, 4, 5, 6].

  ?- append([1, 2, 3], X, [1, 2, 3, 4, 5, 6]).
  X = [4, 5, 6].

  ?- append(X, [4, 5, 6], [1, 2, 3, 4, 5, 6]).
  X = [1, 2, 3].

And the definition of append:

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

Magic!