Find the position of the first bit set in mask, searching from (but not including) nth_bit upwards.
Bits are numbered from 0 (least significant) to sizeof(ulong) * 8 - 1 (31 or 63, usually). To
start searching from the 0th bit, set nth_bit to -1.
| mask |
a ulong containing flags |
| nth_bit |
the index of the bit to start the search from |
|
the index of the first bit set which is higher than |