% From The Art of Eyepl, Chapter 5.
first([Head | _], Head).

contains_item(X, [X | _]).
contains_item(X, [_ | Rest]) :- contains_item(X, Rest).

joins([], Ys, Ys).
joins([X | Xs], Ys, [X | Zs]) :- joins(Xs, Ys, Zs).