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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 iter_files(self, number=-1, etag=None):
 """Iterate over the files associated with this pull request. :param int number: (optional), number of files to return. Default: -1 returns all available files. :param str etag: (optional), ETag from a previous request to the same endpoint :returns: generator of :class:`PullFile <PullFile>`\ s """

        url = self._build_url('files', base_url=self._api)
        return self._iter(int(number), url, PullFile, etag=etag)