private function getEdgeIndex( $sourceNode, $targetNode )
	{
		for( $i=0; $i<sizeof( $this->edges ); $i++ )
		{
			$edge = $this->edges[$i];
			$isSource = $edge->getSourceNode() == $sourceNode;
			$isTarget = $edge->getTargetNode() == $targetNode;
			if( $isSource && $isTarget )
				return $i;
		}
		return -1;
	}