public function isWood()
	{
		if( !$this->hasCycle() )
		{
			$nodes = $this->getNodes();
			foreach( $nodes as $targetSource )
				foreach( $nodes as $source )
					if( $source != $targetSource )
						if( $this->getEntranceGrade( $source, $targetSource ) > 1 )
							return false;
			return true;
		}
		else return false;
	}