code: func (l *List) SetChangedFunc(handler func(index int, mainText string, secondaryText string, shortcut rune)) *List {
  	l.changed = handler
  	return l
  }
docs: // SetChangedFunc sets the function which is called when the user navigates to
  // a list item. The function receives the item's index in the list of items
  // (starting with 0), its main text, secondary text, and its shortcut rune.
  //
  // This function is also called when the first item is added or when
  // SetCurrentItem() is called.