docs: def rotate(self, atom):
          """(atom)->start the cycle at position atom, assumes
          that atom is in the cycle"""
          try:
              index = self.atoms.index(atom)
          except ValueError:
              raise CycleError("atom %s not in cycle"%(atom))
  
          self.atoms = self.atoms[index:] + self.atoms[:index]
          self.bonds = self.bonds[index:] + self.bonds[:index]