code: func (e *OpErr) Error() string {
  	if e == nil {
  		return "<nil>"
  	}
  	s := e.Op
  	if e.Name != "" {
  		s = s + " " + e.Name
  	}
  
  	s = s + ": " + e.Err.Error()
  	if len(e.Refs) > 0 {
  		s = s + " - " + "[" + strings.Join(e.Refs, ", ") + "]"
  	}
  	return s
  }
docs: // Error satisfies the built-in error interface type.