int BN_sub_word(BIGNUM *a, BN_ULONG w)
{
    int i;

    bn_check_top(a);
    w &= BN_MASK2;

    /* degenerate case: w is zero */
    if (!w)
        return 1;
    /* degenerate case: a is zero */
    if (BN_is_zero(a)) {
        i = BN_set_word(a, w);
        if (i != 0)
            BN_set_negative(a, 1);
        return i;
    }
    /* handle 'a' when negative */
    if (a->neg) {
        a->neg = 0;
        i = BN_add_word(a, w);
        a->neg = 1;
        return (i);
    }

    if ((a->top == 1) && (a->d[0] < w)) {
        a->d[0] = w - a->d[0];
        a->neg = 1;
        return (1);
    }
    i = 0;
    for (;;) {
        if (a->d[i] >= w) {
            a->d[i] -= w;
            break;
        } else {
            a->d[i] = (a->d[i] - w) & BN_MASK2;
            i++;
            w = 1;
        }
    }
    if ((a->d[i] == 0) && (i == (a->top - 1)))
        a->top--;
    bn_check_top(a);
    return (1);
}