toggle method
Implementation
List<E> toggle(E value, [bool? exist, int newIndex = 0]) {
final a = use;
exist ??= a.contains(value);
if (exist) {
a.remove(value);
} else {
if (newIndex > -1) {
a.insert(newIndex, value);
} else {
a.add(value);
}
}
return a;
}