big_sub
BIG_ERR_CODE big_sub(BIGNUM *result, BIGNUM *aa, BIGNUM *bb);
if ((err = big_sub(&tmp, &aq, &ap)) != BIG_OK) {
if ((err = big_sub(vmiminus1, vmiminus1, &tmp)) != BIG_OK) {
if ((err = big_sub(veiminus1, veiminus1, &tmp)) != BIG_OK) {
(void) big_sub(&a, &b, &big_One);
(void) big_sub(&a, &c, &big_One);