bn_subw_subw
bn_subw_subw(0, b[0], borrow, &borrow, &r[0]);
bn_subw_subw(a[0], 0, borrow, &borrow, &r[0]);
bn_subw_subw(a[0], b[0], borrow, &borrow, &r);
bn_subw_subw(a[0], b[0] & mask, borrow, &borrow, &r[0]);
bn_subw_subw(a[0], b[0], borrow, &borrow, &r[0]);
bn_subw_subw(a0, b0, borrow, &borrow, &r0);
bn_subw_subw(a1, b1, borrow, &borrow, &r1);
bn_subw_subw(a2, b2, borrow, &borrow, &r2);
bn_subw_subw(a3, b3, borrow, &borrow, &r3);