docs: def _reverse_rounding_method(method):
          """
          Reverse meaning of ``method`` between positive and negative.
          """
          if method is RoundingMethods.ROUND_UP:
              return RoundingMethods.ROUND_DOWN
          if method is RoundingMethods.ROUND_DOWN:
              return RoundingMethods.ROUND_UP
          if method is RoundingMethods.ROUND_HALF_UP:
              return RoundingMethods.ROUND_HALF_DOWN
          if method is RoundingMethods.ROUND_HALF_DOWN:
              return RoundingMethods.ROUND_HALF_UP
          if method in \
             (RoundingMethods.ROUND_TO_ZERO, RoundingMethods.ROUND_HALF_ZERO):
              return method
          raise BasesAssertError('unknown method')