title: Return vowels from word
body: <p>I've got this work to do and my teacher gave me a prolog file with the following facts:</p>
  
  <pre><code>vowel(a).
  vowel(e).
  vowel(i).
  vowel(o).
  vowel(u).
  
  consonant(b).
  consonant(c).
  consonant(d).
  consonant(f).
  consonant(g).
  consonant(h).
  consonant(j).
  consonant(k).
  consonant(l).
  consonant(m).
  consonant(n).
  consonant(p).
  consonant(q).
  consonant(r).
  consonant(s).
  consonant(t).
  consonant(v).
  consonant(w).
  consonant(x).
  consonant(y).
  consonant(z).
  </code></pre>
  
  <p>And I need to create a rule that is able to return the vowels. How can I do that?</p>
  
  <p>The output would be something like this:</p>
  
  <pre><code>blafoo([s,a,r,a], X).
  X = [a].
  </code></pre>
  
  <p>I can't use any prolog predicate.</p>