title: Sum of last two items of list in prolog
body: <p>I'm trying to figure out code for sum of last two item of list. 
  All i got now is function which returns the last item of list :</p>
  
  <pre><code>last_two([],false).
  last_two([H|[]],H).
  last_two([_|T], P) :- last_two(T,P).
  </code></pre>
  
  <p>What should it look like to return this:</p>
  
  <pre><code>?- last_two([4,5,6,3,6,1,7,3],T).
  
  T = 10
  
  ?- last_two([4,5,6,3,6,1,4,5],T).
  
  T = 9
  </code></pre>