title: Prolog length of a list
body: <p>How can I calculate the length of a list</p>
  
  <pre><code>?- size_sub([[b,a,g], [9,3,7,4], [6]],  X).
  X = [3, 4, 1].
  
  ?- size_sub([[c,g,e,w], [7]],  X).
  X = [4, 1].
  
  ?- size_sub([],  X).
  X = [].
  </code></pre>