mpz_perfect_square_p</code> does its tests it calculates a square
     root and then discards it.  For some applications it might be useful to
     return that root.  Suggested by Jason Moxham.
<li> <code>mpz_get_ull</code>, <code>mpz_set_ull</code>,
     <code>mpz_get_sll</code>, <code>mpz_get_sll</code>: Conversions for
     <code>long long</code>.  These would aid interoperability, though a
     mixture of GMP and <code>long long</code> would probably not be too
     common.  Since <code>long long</code>