code: def dummyctrl(self,r,ctrl):
          
          dv = DummyVertex(r)
          dv.view.w,dv.view.h=self.dw,self.dh
          self.grx[dv] = dv
          dv.ctrl = ctrl
          ctrl[r] = dv
          self.layers[r].append(dv)
          return dv
docs: creates a DummyVertex at rank r inserted in the ctrl dict
             of the associated edge and layer.
  
             Arguments:
                r (int): rank value
                ctrl (dict): the edge's control vertices
             
             Returns:
                DummyVertex : the created DummyVertex.