protected static function _arrayFlatten($a, $f = []) {
		if (!$a) {
			return [];
		}
		foreach ($a as $k => $v) {
			if (is_array($v)) {
				$f = static::_arrayFlatten($v, $f);
			} else {
				$f[$k] = $v;
			}
		}
		return $f;
	}