de>SUBC_LIMB</code>.
<li> <code>mpn_sqrtrem2</code>: Use plain variables for <code>sp[0]</code> and
     <code>rp[0]</code> calculations, so the compiler needn't worry about
     aliasing between <code>sp</code> and <code>rp</code>.
<li> <code>mpn_sqrtrem</code>: Some work can be saved in the last step when
     the remainder is not required, as noted in Paul's paper.
<li> <code>mpq_add</code>, <code>mpq_add</code>: The division "op1.den / gcd"
     is done twice, where of course only once is necessary.  Re