mbers in a couple of places.  For some CPUs
     <code>count_trailing_zeros</code> is slow and it's probably worth handling
     the frequently occurring 0 to 2 trailing zeros cases specially.
<li> <code>mpf_t</code> might like to let the exponent be undefined when
     size==0, instead of requiring it 0 as now.  It should be possible to do
     size==0 tests before paying attention to the exponent.  The advantage is
     not needing to set exp in the various places a zero result can arise,
     which avoid