
     dummy length 1 when unwanted, or perhaps an inlined subroutine
     duplicating code under each conditional.  Don't really want to turn
     everything into a dog's dinner just because Cray don't offer an
     <code>alloca</code>.
<li> Cray: <code>mpn_get_str</code> on power-of-2 bases ought to vectorize.
     Does it?  <code>bits_per_digit</code> and the inner loop over bits in a
     limb might prevent it.  Perhaps special cases for binary, octal and hex
     would be worthwhile (very possibly for a