Don't.

wat.ts

/**
 * Magic version of Rx.Observable.sync() that allows for adding more
 * Observables to the input feed while processing.
 *
 * Needs a more descriptive name maybe.
 */
function wat<T,U>(initialState: T, initialFeeds: Rx.IObservable<U>[], iterator: (state: T, e: U) => StateChange<T,U>): any {
  var sub = new Rx.BehaviorSubject(initialState);
  var state = initialState;

  function subscribe(stream: Rx.IObservable<U>): void {
    stream.subscribe(tick, (error) => sub.onError(error));
  }

  function tick(e: U): void {
    var stateChange = iterator(state, e);
    sub.onNext(stateChange.state);
    state = stateChange.state;
    if (stateChange.newStreams) {
      stateChange.newStreams.forEach(subscribe);
    }
  }

  initialFeeds.forEach(subscribe);

  return sub;
}

interface StateChange<T, U> {
  state: T;
  newStreams?: Rx.IObservable<U>[];
}