code: func (v *MessageDialog) FormatSecondaryText(format string, a ...interface{}) {
  	s := fmt.Sprintf(format, a...)
  	cstr := C.CString(s)
  	defer C.free(unsafe.Pointer(cstr))
  	C._gtk_message_dialog_format_secondary_text(v.native(),
  		(*C.gchar)(cstr))
  }
docs: // FormatSecondaryText is a wrapper around
  // gtk_message_dialog_format_secondary_text().