public function previous($index=0)
    {
        if (count($this->sources)>0) {
            return Utility::get($this->sources, $index);
        }
        return null;
    }
public function previous($index=0)
    {
        if (count($this->sources)>0) {
            return Utility::get($this->sources, $index);
        }
        return null;
    }
public function previous ( $ index = 0 ) { if ( count ( $ this -> sources ) > 0 ) { return Utility :: get ( $ this -> sources , $ index ) ; } return null ; }
Get the previous source that send data to this node

This method is very helpful if we want to go trace back to the root
source that is sending data. Since a node can receive data from
multiple sources, the $index params help to return the node you need.
By default $index is 0 meaning the first node that streams data to this
node.

@param integer $index The index of the source that you want to get

@return Node The source that sends data to this node
Get the previous source that send data to this node