public function toTable( $showNull = false)
	{
		$heading = "";
		$t = "<table class='filledframe' cellpadding=2 cellspacing=0>";
		$nodes = $this->getNodes();
		for( $j=0; $j<$this->getNodeSize(); $j++ )
		{
			$target = $nodes[$j];
			$heading .= "<th width=20>".$target->getNodeName()."</th>";
		}
		$t .= "<tr><th></th>".$heading."</tr>";
		for( $i=0; $i<$this->getNodeSize(); $i++ )
		{
			$source = $nodes[$i];
			$line = "";
			for( $j=0; $j<$this->getNodeSize(); $j++ )
			{

				$target = $nodes[$j];
				if( $this->isEdge( $source, $target ) )
					$value = $this->getEdgeValue( $source, $target );
				else if( $showNull )
					$value = 0;
				else
					$value = "";
				$line .= "<td align=center>".$value."</td>";
			}
			$t .= "<tr><th width=20>".$source->getNodeName()."</th>".$line."</tr>";
		}
		$t .= "</table>";
		return $t;
	}