code: def next(self):
          
          if self._mode != "r":
              raise UnsupportedOperation("not available in 'w' mode")
  
          self._n += 1
          if self._n > self._nb_markers:
              raise StopIteration()
  
          return self._bim.index[self._n - 1], self._read_current_marker()
docs: Returns the next marker.
  
          Returns:
              tuple: The marker name as a string and its genotypes as a
              :py:class:`numpy.ndarray`.