bn_sub_words
borrow = bn_sub_words(r, a, b, min_len);
bn_sub_words(wnum.d, wnum.d, sdiv->d, div_n);
if (bn_sub_words(wnum.d, wnum.d, tmp->d, div_n + 1)) {
BN_ULONG bn_sub_words(BN_ULONG *r, const BN_ULONG *a, const BN_ULONG *b,
borrow = bn_sub_words(r, a, b, n);
mask = carry - bn_sub_words(r, a, n, n_len);
mask = bn_ct_ne_zero(tp[n_len]) - bn_sub_words(rp, tp, np, n_len);