protected void add(Edge edge){
  edges.add(edge);
  add(edge.getDirEdge(0));
  add(edge.getDirEdge(1));
}

protected void add ( Edge edge ) { edges . add ( edge ) ; add ( edge . getDirEdge ( 0 ) ) ; add ( edge . getDirEdge ( 1 ) ) ; }
adds the edge and its directededges with this planargraph . assumes that the edge has already been created with its associated directedges . only subclasses can add edges , to ensure the edges added are of the right class .
