code: def reduce_vertex(self, name1, *names):
          
          v = self.vertices[name1]
          for n in names:
              w = self.vertices[n]
              v.alias.update(w.alias)
              # replace mapping from n w by to v
              self.vertices[n] = v
docs: treat name1, name2, ... as same point.
  
          name2.alias, name3.alias, ... are merged with name1.alias
          the key name2, name3, ... in self.vertices are kept and mapped to
          same Vertex instance as name1