mirror(leaf(Label), leaf(Label)).
mirror(tree(B1, B2), tree(B3, B4)) :- mirror(B2, B3), mirror(B1, B4).
/* Tests */
test_answer1 :-
mirror(leaf(foo), leaf(foo)),
write(‘OK’).
test_answer2 :-
mirror(tree(leaf(foo), leaf(bar)), tree(leaf(bar), leaf(foo))),
write(‘OK’).
test_answer3 :-
mirror(tree(tree(leaf(1), leaf(2)), leaf(4)), T),
write(T).
test_answer4 :-
mirror(tree(tree(leaf(1), leaf(2)), tree(leaf(3), leaf(4))), T),
write(T).