code: func (e frame) String() string {
  	if e.pc == 0 {
  		return "unknown.unknown:0"
  	}
  	f := runtime.FuncForPC(e.pc)
  	if f == nil {
  		return "unknown.unknown:0"
  	}
  	file, line := f.FileLine(e.pc)
  	return fmt.Sprintf("%s:%s:%d", f.Name(), filepath.Base(file), line)
  }
docs: // String returns a human readable form of the frame.