code: func Errorf(format string, args ...interface{}) error {
  	return &fundamental{
  		msg:   fmt.Sprintf(format, args...),
  		stack: callers(),
  	}
  }
docs: // Errorf formats according to a format specifier and returns the string
  // as a value that satisfies error.
  // Errorf also records the stack trace at the point it was called.