docs: def get_negative(self, cls=None, **kwargs):
          """Returns a generator that generates negative cases by
          "each negative value in separate case" algorithm.
          """
          for attr, set_of_values in kwargs.iteritems():
              defaults = {key: kwargs[key][-1]["default"] for key in kwargs}
              defaults.pop(attr)
              for value in set_of_values[:-1]:
                  case = cls() if cls else self._CasesClass()
                  setattr(case, attr, value)
                  for key in defaults:
                      setattr(case, key, defaults[key])
                  yield case