AddedSubscriber[T] = (T) -> void

RemovedSubscriber[T] = (T) -> void

ObservableCollection[T] = (AddedSubscriber[T], RemovedSubscriber[T]) -> void

CollectionSubject[T] = ref object
  source*: ObservableCollection[T]
  values*: seq[T]
  addedSubscribers: seq[AddedSubscriber[T]]
  removedSubscribers: seq[RemovedSubscriber[T]]

proc observableCollection[T](values: seq[T] = @[]): CollectionSubject[T]

proc add[T](self: CollectionSubject[T]; item: T): void

proc remove[T](self: CollectionSubject[T]; item: T): void

proc map[T, R](self: ObservableCollection[T]; mapper: (T) -> R): ObservableCollection[R]

proc observableCollection[T](source: ObservableCollection[T]): CollectionSubject[T]

proc combineLatest[A, B, R](a: ObservableCollection[A]; b: ObservableCollection[B];
                         mapper: (A, B) -> R): ObservableCollection[R]

template map[T; R](self: CollectionSubject[T]; mapper: (T) -> R): ObservableCollection[R]