Reverse meaning of ``method`` between positive and negative.

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')