Source: The Art of Prolog
Program source code: tree_member.pl
/* tree_member(Element,Tree):- Element is an element of the binary tree Tree */ tree_member(X,tree(X,Left,Right)). tree_member(X,tree(Y,Left,Right)) :- tree_member(X,Left). tree_member(X,tree(Y,Left,Right)) :- tree_member(X,Right). % Program 3.24: Testing tree membership