code: func (u *UI) Warn(format string, args ...interface{}) {
  	var w io.Writer = readline.Stdout
  	icon := u.BoldColorString(Yellow, "⚠ ")
  	if !Attached() {
  		w = readline.Stderr
  		icon = ""
  	}
  
  	o := fmt.Sprintf(icon+format, args...)
  	fmt.Fprintln(w, ansiwrap.WrapIndent(o, u.Cols, u.Indent, u.Indent))
  }
docs: // Warn handles outputting warning information to the user such as
  // messages about needing to be logged in.
  //
  // The warning is printed out to stderr if stdout is not attached to a
  // terminal.