leading_zeros</code>, though it's possible an already
     normalized operand might not be encountered very often.
<li> Rewrite <code>umul_ppmm</code> to use floating-point for generating the
     most significant limb (if <code>GMP_LIMB_BITS</code> &lt= 52 bits).
     (Peter Montgomery has some ideas on this subject.)
<li> Improve the default <code>umul_ppmm</code> code in longlong.h: Add partial
     products with fewer operations.
<li> Consider inlining <code>mpz_set_ui</code>.  This would be both small 