gmp</code>.  This establishes good cross-checks, but it might be
     better to use simple reference routines where possible.  Where it's not
     possible some attention could be paid to the order of the tests, so a
     <code>libgmp</code> routine is only used for tests once it seems to be
     good.
<li> <code>MUL_FFT_THRESHOLD</code> etc: the FFT thresholds should allow a
     return to a previous k at certain sizes.  This arises basically due to
     the step effect caused by size multiples effectively