ssible to say at most one leading zero,
     which is what both it and <code>mpz_get_str</code> currently do.  But
     want to be careful not to bind ourselves to something that might not suit
     another implementation.
<li> <code>va_arg</code> doesn't do the right thing with <code>mpz_t</code>
     etc directly, but instead needs a pointer type like <code>MP_INT*</code>.
     It'd be good to show how to do this, but we'd either need to document
     <code>mpz_ptr</code> and friends, or perhaps fallback 