class JunkComparisonException (BaseException):

    def __str__ (self):
        return "Attempting to perform calculations involving junk values."

class Junk:

    def __init__ (self, represents: str = None):
        assert represents is None or isinstance(represents, str)
        self.repr = represents

    def __str__ (self):
        if self.repr:
            return '['+self.repr+']'
        return "[junk]"

    def __repr__ (self):
        return self.__str__()

    def __add__ (self, other):
        return self

    def __radd__ (self, other):
        return self

    def __sub__ (self, other):
        return self

    def __rsub__ (self, other):
        return self