title: Prolog verification of a binary trees
body: <p>I am a beginner with prolog and I want to write a predicate that turns true if the argument is a tree. I have this code but it gives me always false. can anyone help me pls.</p>
  
  <pre><code>arb_true(nil).
  arb_true([X,G,D]):- X=[_,G,D], arb_true(G), arb_true(D).
  </code></pre>
  
  <p>the query is <code>arb_true([6,[4,[1,[],[]],[]],[9,[],[]]]).</code></p>