private function _getCriteriaKeys( array $prefix, MW_Common_Criteria_Expression_Interface $expr = null )
	{
		if( $expr === null ) { return array(); }

		$result = array();

		foreach( $this->_getCriteriaNames( $expr ) as $item )
		{
			if( ( $pos = strpos( $item, '(' ) ) !== false ) {
				$item = substr( $item, 0, $pos );
			}

			if( ( $pos = strpos( $item, ':' ) ) !== false ) {
				$item = substr( $item, $pos + 1 );
			}

			$result = array_merge( $result, $this->_cutNameTail( $prefix, $item ) );
		}

		return $result;
	}