code: def successors(self):
          
          if not self.children:
              return
          for part in self.children:
              # Parts local to the current Compound.
              yield part
              # Parts further down the hierarchy.
              for subpart in part.successors():
                  yield subpart
docs: Yield Compounds below self in the hierarchy.
  
          Yields
          -------
          mb.Compound
              The next Particle below self in the hierarchy