code: func (m *subMenu) Append(i *MenuItem) (err error) {
  	if err = m.isActionable(); err != nil {
  		return
  	}
  	if _, err = synchronousEvent(m.c, m, m.w, Event{Name: EventNameSubMenuCmdAppend, TargetID: m.id, MenuItem: i.toEvent()}, EventNameSubMenuEventAppended); err != nil {
  		return
  	}
  	m.items = append(m.items, i)
  	return
  }
docs: // Append appends a menu item into the sub menu