bmul_number
bmul_number(r, a, b, bmachine.scale);
bmul_number(a, a, a, ascale);
bmul_number(a, a, a, ascale);
bmul_number(r, r, a, mscale);
void bmul_number(struct number *, struct number *,
bmul_number(fract_part, fract_part, num_base,