docs: def add_input(self, in_name, type_or_parse=None):
          """ Declare a possible input
          """
          if type_or_parse is None:
              type_or_parse = GenericType()
          elif not isinstance(type_or_parse, GenericType) and callable(type_or_parse):
              type_or_parse = GenericType(parse=type_or_parse)
          elif not isinstance(type_or_parse, GenericType):
              raise ValueError("the given 'type_or_parse' is invalid")
          self._inputs[in_name] = type_or_parse