public function getGrade( $source, $target )
	{
		if( $this->isEdge( $source, $target ) )
			return 1;
		$nodes = $this->getTargetNodes( $source );
		foreach( $nodes as $node )
		{
			$way = $this->getGrade( $node, $target );
			return ++$way;
		}
		return false;
	}