code: def contains(self, key_or_keypath):
          
          if isinstance(key_or_keypath, list):
              if len(key_or_keypath) == 0:
                  # empty list is root
                  return False
              val = self 
              next_key = None
              for next_key in key_or_keypath:
                  if next_key in val:
                      val = val[next_key]
                  else:
                      return False
              return True
          else:
              return key_or_keypath in self.__dict__['p']
docs: Allows the 'in' operator to work for checking if a particular key (or keypath)
          is inside the dictionary.