title: Multiplication of all nodes in a binary tree
body: <p>I'm trying to multiply all nodes in a binary tree
  but I'm only getting 0 as a result. My code is:</p>
  <pre><code>product_tree(nil,0).
  product_tree(t(L,X,R), N):-
     product_tree(L,L1),
     product_tree(R,R2),
     N is X * L1 * R2.
  </code></pre>