// refill A+B, read A, read B than refill A, read A, refill B, read B.
static inline void possibly_refill(struct bit_source* source, unsigned num_bits)
{
// refill A+B, read A, read B than refill A, read A, refill B, read B.
static inline void possibly_refill(struct bit_source* source, unsigned num_bits)
{