anchor.diff:              (subscriptionAddress, unsubscribe, world)
           { ObserverAddress = stream.ObserverAddress; Subscribe = subscribe }
   
  +    /// TODO: document!
       let track2
           (tracker : 'a -> Event<'a, 'o> -> World -> 'a * bool)
           (stream : EventStream<'a, 'o>) :
  
positive.code:     let updateOptSimulantValue valueSetter stream =
          subscribe (fun a w -> (Cascade, World.updateOptSimulant (valueSetter a w) a.SubscriberAddress w)) stream
  
      let worldStateValue (valueGetter : WorldState -> 'b) (stream : EventStream<WorldStateChangeData, 'o>) =
          filter (fun a w ->
              let oldValue = valueGetter a.Data.OldWorldState
  
negative.code:             (subscriptionAddress, unsubscribe, world)
          { ObserverAddress = stream.ObserverAddress; Subscribe = subscribe }
  
      /// TODO: document!
      let track
          (tracker : 'b -> World -> 'b * bool)
          (state : 'b)
  