and if that
     somehow succeeds then an <code>abort</code>.  Various size overflows of
     this kind are not handled gracefully, probably resulting in segvs.
     <br>
     In <code>mpz_n_pow_ui</code>, detect when the count of low zero bits
     exceeds an <code>unsigned long</code>.  There's a (small) chance of this
     happening but still having enough memory to represent the value.
     Reported by Winfried Dreckmann in for instance <code>mpz_ui_pow_ui (x,
     4UL, 1431655766UL)</code>.
<li> <code>