docs: def _index_file(self):
          """Open and index the contour file
  
          This function populates the internal list of contours
          as strings which will be available as `self.data`.
          """
          with self.filename.open() as fd:
              data = fd.read()
  
          ident = "Contour in frame"
          self._data = data.split(ident)[1:]
          self._initialized = True