 live in lshift.asm and omit rshift.asm on finding
     <code>mpn_rshift</code> already provided.
<li> Cray T3E: Experiment with optimization options.  In particular,
     -hpipeline3 seems promising.  We should at least up -O to -O2 or -O3.
<li> Cray: <code>mpn_com</code> and <code>mpn_and_n</code> etc very probably
     wants a pragma like <code>MPN_COPY_INCR</code>.
<li> Cray vector systems: <code>mpn_lshift</code>, <code>mpn_rshift</code>,
     <code>mpn_popcount</code> and <code>mpn_hamdist</code> are 