d.  If it does actually exist
     somewhere it would very likely be an improvement over a function call to
     umul.asm.
<li> <code>mpn_get_str</code> final divisions by the base with
     <code>udiv_qrnd_unnorm</code> could use some sort of multiply-by-inverse
     on suitable machines.  This ends up happening for decimal by presenting
     the compiler with a run-time constant, but the same for other bases would
     be good.  Perhaps use could be made of the fact base&lt;256.
<li> <code>mpn_umul_ppmm</