/code>'s into a single loop stopping when
     the end of one operand is reached (and then looking for a non-zero in the
     rest of the other).
<li> <code>mpf_mul_2exp</code>, <code>mpf_div_2exp</code>: The use of
     <code>mpn_lshift</code> for any size&lt;=prec means repeated
     <code>mul_2exp</code> and <code>div_2exp</code> calls accumulate low zero
     limbs until size==prec+1 is reached.  Those zeros will slow down
     subsequent operations, especially if the value is otherwise only small.
    