docs: def GetTopicsTree(self):
          '''Reads and returns the topics tree.
          This auxiliary function reads and returns the topics tree file
          contents for the CHM archive.
          '''
          if self.topics is None:
              return None
  
          if self.topics:
              res, ui = chmlib.chm_resolve_object(self.file, self.topics)
              if (res != chmlib.CHM_RESOLVE_SUCCESS):
                  return None
  
          size, text = chmlib.chm_retrieve_object(self.file, ui, 0l, ui.length)
          if (size == 0):
              sys.stderr.write('GetTopicsTree: file size = 0\n')
              return None
          return text