code: func (s *stRingBuffer) NthPreviousEntry(n int) (value string, ok bool) {
  	if n >= s.size {
  		return "", false
  	}
  	index := s.head - n
  	if index < 0 {
  		index += s.max
  	}
  	return s.entries[index], true
  }
docs: // NthPreviousEntry returns the value passed to the nth previous call to Add.
  // If n is zero then the immediately prior value is returned, if one, then the
  // next most recent, and so on. If such an element doesn't exist then ok is
  // false.