clause_tree(true) :- !.      /* true leaf */ 
clause_tree((G,R)) :- 
   !, 
   clause_tree(G),
   clause_tree(R).           /* search each branch */ 
clause_tree(G) :- 
   clause(G,Body),
   clause_tree(Body).        /* grow branches */ 

member(X,[X|_]). 
member(X,[_|R]) :- member(X,R).