, suppose a function <code>string_of_int : int -> string</code>,
    that takes an integer and generates the string that represent this
    integer. Applied to <code>tree_map</code> and a tree of integers
    (i.e. of type <code>int tree</code>), it would yield a tree of strings
    (i.e. of type <code>string tree</code>).
  </li>
</ol>
