<ol>
  <li>
   Write a function <code>is_sorted : string array -> bool</code> which
   checks if the values of the input array are sorted in
   strictly increasing order, implying that its elements are unique
   (use <code>String.compare</code>).
  </li>
  <li>
    Using the
    <a href="https://en.wikipedia.org/wiki/Binary_search_algorithm">
      binary search algorithm</a>, an element can be found very
    quickly in a sorted array.
    
    <br/>

    Write a function <code>find : string array -> string -> int</code> such that
   <code>find arr word</code> is the index of the <code>word</code> in
   the sorted array <code>arr</code> if it occurs in <code>arr</code>
   or -1 if <code>word</code> does not occur in <code>arr</code>.

   <br/>

   The number or array accesses will be counted, to check that you
   obtain the expected algorithmic complexity. Beware that you really
   perform the minimal number of accesses. For instance, if your
   function has to test the contents of a cell twice, be sure to put
   the result of the access in a variable, and then perform the tests
   on that variable.
  </li>
</ol>
