Find the position of the first bit set in
from (but not including)
nth_bit upwards. Bits are numbered
from 0 (least significant) to sizeof(#gulong) * 8 - 1 (31 or 63,
usually). To start searching from the 0th bit, set
nth_bit to -1.
A #gulong containing flags.
The index of the bit to start the search from.