Traits instead of interfaces?

[Doom2fan]

We could have traits instead of interfaces, while still allowing "trait inheritance". This would mean classes and structs can explicitly implement a trait internally, while also allowing external "extension" of types with traits. e.g.

trait Iterator!(T, TState) {
    TState InitialState ();
    T? TryGetNext (ref TState state);
}

extends IList!(T) with Iteratable!(T, intN) {
    intN InitialState => 0;
    T? TryGetNext (ref intN index) => index < length ? this[index] : null;
}

class ClassWithIterator : Iterator<T, int32> {
    // [...]
}