docs: def get_pairwise(self, cls=None, **kwargs):
          """Returns a generator that generates positive cases by
          "pairwise" algorithm.
          """
          for set_of_values in allpairs(kwargs.values()):
              case = cls() if cls else self._CasesClass()
              for attr, value in izip(kwargs.keys(), set_of_values):
                  setattr(case, attr, value)
              yield case