_bit_make_mask
*_bitstr |= _bit_make_mask(_start, _stop);
*_bitstr |= _bit_make_mask(_start, _BITSTR_BITS - 1);
*_stopbitstr |= _bit_make_mask(0, _stop);
*_bitstr &= ~_bit_make_mask(_start, _stop);
*_bitstr &= ~_bit_make_mask(_start, _BITSTR_BITS - 1);
*_stopbitstr &= ~_bit_make_mask(0, _stop);
_test &= _bit_make_mask(_start, _BITSTR_BITS - 1);
_test |= _bit_make_mask(0, _start - 1);
mask = _bit_make_mask(_start, _bit_offset(curbitstr_len - 1));
mask = _bit_make_mask(0, _bit_offset(_nbits - 1));