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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 comments(self):
 # pylint: disable=E0202 """Return forest of comments, with top-level comments as tree roots. May contain instances of MoreComment objects. To easily replace these objects with Comment objects, use the replace_more_comments method then fetch this attribute. Use comment replies to walk down the tree. To get an unnested, flat list of comments from this attribute use helpers.flatten_tree. """

        if self._comments is None:
            self.comments = Submission.from_url(  # pylint: disable=W0212
                self.reddit_session, self._api_link, comments_only=True)
        return self._comments