The predicate member/2 verifies if an element is in a list.
	     	?- member(b,[a,v,b,c]). 
	      	true. 
		?- member(a,[b,c,g]).
		false.
		?- member(X,[a,b,c]).
		X = a ;  
		X = b ;
		X = c ;
		false.
		| ?- member(X,[a,b,c]),write(X),nl,fail.
		a
		b
		c
		false.

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