?- nr_vowel([],X).
             X = 0
             ?- nr_vowel([a,r,e,d,i],X).
             X = 3
             ?- nr_vowel([m,r],X).
             X = 0
	     ?- nr_vowel([s,e,e,d],X).
             X = 2


             vowel(X):- member(X,[a,e,i,o,u]).
 
             nr_vowel([],0).
             nr_vowel([X|T],N):- vowel(X),nr_vowel(T,N1),N is N1+1,!.
             nr_vowel([X|T],N):- nr_vowel(T,N).