de>[[1];[2];[3]]</code>
  </li>
  <li>
    Consider the definition of the type <code>tree</code> given in the prelude.
    It represents binary trees carrying data items, on its internal nodes, and on its
    leaves.

    <br />

    Write a function <code>tree_map : ('a -> 'b) -> 'a tree ->
    'b tree</code> such that <code>tree_map f t</code> yields a tree of the same
    structure as <code>t</code>, but with all its data values <code>x</code>
    replaced by <code>f x</code>

    <br />

    For example