code: def get_vertex_faces(self):
          
          if self._vertex_faces is None:
              self._vertex_faces = [[] for i in xrange(len(self.get_vertices()))]
              for i in xrange(self._faces.shape[0]):
                  face = self._faces[i]
                  for ind in face:
                      self._vertex_faces[ind].append(i)
          return self._vertex_faces
docs: List mapping each vertex index to a list of face indices that use it.