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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 exists(self):
 """ Retrieves whether the document exists in the remote database or not. :returns: True if the document exists in the remote database, otherwise False """

        if '_id' not in self or self['_id'] is None:
            return False

        resp = self.r_session.head(self.document_url)
        if resp.status_code not in [200, 404]:
            resp.raise_for_status()

        return resp.status_code == 200