code: func source(lines [][]byte, n int) []byte {
  	n-- // in stack trace, lines are 1-indexed but our array is 0-indexed
  	if n < 0 || n >= len(lines) {
  		return dunno
  	}
  	return bytes.TrimSpace(lines[n])
  }
docs: // source returns a space-trimmed slice of the n'th line.