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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def filter(self, filter_fn):
 """Applies a filter to the stream. Attributes: filter_fn (function):
 The user-defined filter function. """

        op = Operator(
            _generate_uuid(),
            OpType.Filter,
            "Filter",
            filter_fn,
            num_instances=self.env.config.parallelism)
        return self.__register(op)