ode>mpz_xor</code> should
     use <code>mpn_and_n</code> etc for the benefit of the small number of
     targets with native versions of those routines.  Need to be careful not to
     pass size==0.  Is some code sharing possible between the <code>mpz</code>
     routines?
<li> <code>mpf_add</code>: Don't do a copy to avoid overlapping operands
     unless it's really necessary (currently only sizes are tested, not
     whether r really is u or v).
<li> <code>mpf_add</code>: Under the check for v having no