de> conversion and free.  Perhaps
          <code>mpz_get_str</code> can write directly into a
          <code>string</code>, to avoid copying.
          <br>
          Consider making such <code>string</code> returning variants
          available for use with plain <code>mpz_t</code> etc too.
     </ul>
</ul>

<h4>Miscellaneous</h4>
<ul>
<li> <code>mpz_gcdext</code> and <code>mpn_gcdext</code> ought to document
     what range of values the generated cofactors can take, and preferably
     ensure the defi