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