code: def append_file(self, fs_entry):
          
  
          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
docs: 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