def METHOD_NAME(self):
    try:
        return self.lhs.METHOD_NAME.union(self.rhs.METHOD_NAME)
    except AttributeError:
        pass
    try:
        return self.lhs.METHOD_NAME
    except AttributeError:
        pass
    try:
        return self.rhs.METHOD_NAME
    except AttributeError:
        return frozenset()
    else:
        TypeError('Failed to retrieve symbolic functions')