% magic sets append with mode (+, +, -). !go-append (cons X Xs) Ys -o !go-append Xs Ys. !go-append nil Ys -o !append nil Ys Ys. !go-append (cons X Xs) Ys, !append Xs Ys Zs -o !append (cons X Xs) Ys (cons X Zs). %exec * !go-append (cons 1 (cons 2 nil)) (cons 3 (cons 4 nil)).