code: func Errorf(format string, args ...interface{}) (err error) {
  	err = fmt.Errorf(format, args...)
  	trace := wrapWithDepth(err, 2)
  	trace.AddUserMessage(format, args...)
  	return trace
  }
docs: // Errorf is similar to fmt.Errorf except that it captures
  // more information about the origin of error, such as
  // callee, line number and function that simplifies debugging