code: func (l *lexer) linePosition() int {
  	lb := strings.LastIndex(l.input[:l.lastPos], "\n")
  	if lb == -1 {
  		return 1 + int(l.lastPos)
  	}
  	return 1 + int(l.lastPos) - lb
  }
docs: // linePosition reports at which character in the current line
  // we are on.