/**
  * Create a new directed edge inside the given graph
  * @param SimpleGraph $oGraph
  * @param string $sId The unique identifier of this edge in the graph
  * @param GraphNode $oSourceNode
  * @param GraphNode $oSinkNode
  */
 public function __construct(SimpleGraph $oGraph, $sId, GraphNode $oSourceNode, GraphNode $oSinkNode)
 {
     parent::__construct($sId);
     $this->oSourceNode = $oSourceNode;
     $this->oSinkNode = $oSinkNode;
     $oGraph->_AddEdge($this);
 }