bitstr_size
calloc(bitstr_size(nbits), sizeof(bitstr_t))
((name)[bitstr_size(nbits)])
(bitstr_t *)calloc((size_t)bitstr_size(nbits), sizeof(bitstr_t))
((name)[bitstr_size(nbits)])
int i = bitstr_size(n);
i, _bit_byte(i), _bit_mask(i), bitstr_size(i));