code: func (s *Selection) AddBackFiltered(selector string) *Selection {
  	return s.AddSelection(s.prevSel.Filter(selector))
  }
docs: // AddBackFiltered reduces the previous set of elements on the stack to those that
  // match the selector string, and adds them to the current set.
  // It returns a new Selection object containing the current Selection combined
  // with the filtered previous one