sparsebit_is_clear
bool sparsebit_is_clear(const struct sparsebit *sbit, sparsebit_idx_t idx);
if (!sparsebit_is_clear(s, idx))
assert(sparsebit_is_clear(s, idx));
assert(!sparsebit_is_clear(s, first));
assert(sparsebit_is_clear(s, first));
assert(sparsebit_is_clear(s, first) == !get_value(first));
assert(sparsebit_is_clear(s, first) || next <= last);