docs: def _check_states_enum(cls):
          """Check if states enum exists and is proper one."""
          states_enum_name = cls.context.get_config('states_enum_name')
          try:
              cls.context['states_enum'] = getattr(
                  cls.context.new_class, states_enum_name)
          except AttributeError:
              raise ValueError('No states enum given!')
  
          proper = True
          try:
              if not issubclass(cls.context.states_enum, Enum):
                  proper = False
          except TypeError:
              proper = False
  
          if not proper:
              raise ValueError(
                  'Please provide enum instance to define available states.')