<p>
  A binary tree <code>t</code>, of the <code>'a bt</code> type given
  in the prelude, is either an empty tree, or the root of a tree with
  a value and two children subtrees.
</p>

<ol>
  <li>
    Write a function <code>height : 'a bt -> int</code> that computes
    the height of a tree.
  </li>
  <li>
    A tree is <em>balanced</em> if, for all internal
    node <code>n</code>, its two subtrees have the same height.  Write
    a function <code>balanced : 'a bt -> bool</code> that tells if a
    tree i