docs: def seek(self, n):
          """Gets to a certain marker position in the BED file.
  
          Args:
              n (int): The index of the marker to seek to.
  
          """
          if self._mode != "r":
              raise UnsupportedOperation("not available in 'w' mode")
  
          if 0 <= n < self._nb_markers:
              self._n = n
              self._bed.seek(self._get_seek_position(n))
  
          else:
              # Invalid seek value
              raise ValueError("invalid position in BED: {}".format(n))