public function hasDirected()
    {
        foreach ($this->set->getEdges() as $edge) {
            if ($edge instanceof EdgeDirected) {
                return true;
            }
        }

        return false;
    }
public function hasDirected()
    {
        foreach ($this->set->getEdges() as $edge) {
            if ($edge instanceof EdgeDirected) {
                return true;
            }
        }

        return false;
    }
public function hasDirected ( ) { foreach ( $ this -> set -> getEdges ( ) as $ edge ) { if ( $ edge instanceof EdgeDirected ) { return true ; } } return false ; }
checks whether the graph has any directed edges

This method is intentionally not named "isDirected()" (aka digraph),
because that might be misleading in regards to empty and/or mixed graphs.

@return boolean
checks whether the graph has any directed edges