<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def maybe_a(generator):
 """ Generates either an arbitrary value of the specified generator or None. This is a class factory, it makes a class which is a closure around the specified generator. """


    class MaybeAGenerator(ArbitraryInterface):
        """
        A closure class around the generator specified above, which generates
        either that generator or None.
        """

        @classmethod
        def arbitrary(cls):
            """
            Generate either the enclosed generator or None.
            """
            return arbitrary(one_of(None, generator))

    MaybeAGenerator.__name__ = ''.join(['maybe_a(', generator.__name__, ')'])
    return MaybeAGenerator