docs: def get_timemachine_instance(self, key):
          """Return a TimeMachine for a related object.
  
          Take a string argument representing a ForeignKey field name, find what
          object was related to this one at the time of this TimeMachine and 
          return a TimeMachine for that related object.
  
          """
          modcommit = self._get_modcommit(key)
          if not modcommit: 
              return None
          return TimeMachine(uid = modcommit.value)