code: def setdummies(self,e):
          
          v0,v1 = e.v
          r0,r1 = self.grx[v0].rank,self.grx[v1].rank
          if r0>r1:
              assert e in self.alt_e
              v0,v1 = v1,v0
              r0,r1 = r1,r0
          if (r1-r0)>1:
              # "dummy vertices" are stored in the edge ctrl dict,
              # keyed by their rank in layers.
              ctrl=self.ctrls[e]={}
              ctrl[r0]=v0
              ctrl[r1]=v1
              for r in xrange(r0+1,r1):
                  self.dummyctrl(r,ctrl)
docs: creates and defines all needed dummy vertices for edge e.