docs: async def health_check(self) -> Iterator[HealthCheckFail]:
          """
          Perform the checks. So far:
  
          - Make a list of the unique destination states from the transitions
            list, then check the health of each of them.
          """
  
          ds_class = getattr(settings, 'DEFAULT_STATE', '')
          forbidden_defaults = [None, '', 'bernard.engine.state.DefaultState']
  
          if ds_class in forbidden_defaults:
              yield HealthCheckFail(
                  '00005',
                  f'Default state (`DEFAULT_STATE` in settings) is not set. '
                  f'You need to set it to your own implementation. Please refer '
                  f'yourself to the doc. See '
                  f'https://github.com/BernardFW/bernard/blob/develop/doc/'
                  f'get_started.md#statespy'
              )
  
          try:
              import_class(ds_class)
          except (ImportError, KeyError, AttributeError, TypeError):
              yield HealthCheckFail(
                  '00005',
                  f'Cannot import "{ds_class}", which is the value'
                  f' of `DEFAULT_STATE` in the configuration. This means either'
                  f' that your `PYTHONPATH` is wrong or that the value you gave'
                  f' to `DEFAULT_STATE` is wrong. You need to provide a default'
                  f' state class for this framework to work. Please refer'
                  f' yourself to the documentation for more information. See'
                  f' https://github.com/BernardFW/bernard/blob/develop/doc/'
                  f'get_started.md#statespy'
              )
  
          states = set(t.dest for t in self.transitions)
  
          for state in states:
              async for check in state.health_check():
                  yield check