public function iterateParts(callable $callable)
    {
        $parts = [];
        foreach ($this->_conditions as $k => $c) {
            $key =& $k;
            $part = $callable($c, $key);
            if ($part !== null) {
                $parts[$key] = $part;
            }
        }
        $this->_conditions = $parts;

        return $this;
    }