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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 append_file(self, fs_entry):
 """ Adds an FSEntry object to this METS document's tree. Any of the represented object's children will also be added to the document. A given FSEntry object can only be included in a document once, and any attempt to add an object the second time will be ignored. :param metsrw.mets.FSEntry fs_entry: FSEntry to add to the METS document """


        if fs_entry in self._root_elements:
            return
        self._root_elements.append(fs_entry)
        # Reset file lists so they get regenerated with the new files(s)
        self._all_files = None