FIRST_DIGIT
if (MP_DIGIT(r, FIRST_DIGIT) & 0x200) {
MP_DIGIT(r,FIRST_DIGIT) &= 0x1FF;
for (i = FIRST_DIGIT; i < MP_USED(a)-1; i++) {
s1[i-FIRST_DIGIT] = (MP_DIGIT(a, i) >> 9)
s1[i-FIRST_DIGIT] = MP_DIGIT(a, i) >> 9;
MP_DIGIT(r,FIRST_DIGIT) &= 0x1FF;