code: func Caller() string {
  	_, file, line, ok := runtime.Caller(2)
  	if !ok {
  		return "?"
  	}
  
  	rel, err := filepath.Rel(RootPath(), file)
  	if err != nil {
  		return "?"
  	}
  
  	return fmt.Sprintf("%s:%d", rel, line)
  }
docs: // Caller returns a caller's call location. If F1 calls F2 which in its turn
  // calls Caller, then this function will return a location within F1 where it
  // calls F2.