public function clear(string $primary_key, $sub_key = null, int $delay = 0): bool
    {
        if (!$this->enabled) {
            return false; // Not possible.
        }
        if (!isset($sub_key)) {
            $key = $this->nspKey($primary_key);
        } else {
            $key = $this->key($primary_key, $sub_key);
        }
        return $key && $this->Pool->delete($key, $delay);
    }