anchor.diff:              oldValue <> newValue)
               stream
   
  +    /// Make an event stream from an observer address and change events from the world's state.
       let observeWorldStateValue valueGetter observerAddress =
           observe WorldStateChangeEventAddress observerAddress |>
           worldStateValue valueGetter
   
  +    /// Make an event stream from an observer address and one change event from the world's state
  +    /// per frame.
       let observeWorldStateValueCyclic valueGetter observerAddress =
           observeWorldStateValue valueGetter observerAddress |>
           noMoreThanOncePerTick
   
  +    /// Make an event stream from an observer address and the observer's change events.
       let observeSimulantValue (valueGetter : 'a -> 'b) (simulantAddress : 'a Address) (observerAddress : 'o Address) =
           let changeEventAddress = Address.changeType<Simulant SimulantChangeData, 'a SimulantChangeData> SimulantChangeEventAddress ->>- simulantAddress
           observe changeEventAddress observerAddress |>
           simulantValue valueGetter
   
  +    /// Make an event stream from an observer address and one of the observer's change events per
  +    /// frame.
       let observeSimulantValueCyclic valueGetter simulantAddress observerAddress =
           observeSimulantValue valueGetter simulantAddress observerAddress |>
           noMoreThanOncePerTick
  
positive.code:             (subscriptionAddress, unsubscribe, world)
          { ObserverAddress = stream.ObserverAddress; Subscribe = subscribe }
  
      let track
          (tracker : 'b -> World -> 'b * bool)
          (state : 'b)
  
negative.code:             (subscriptionAddress'', unsubscribe, world)
          { ObserverAddress = stream.ObserverAddress; Subscribe = subscribe }
  
      /// Filter an event stream's events by 'pred'.
      let filter (pred : Event<'a, 'o> -> World -> bool) (stream : EventStream<'a, 'o>) =
          let subscribe = fun world ->
              let subscriptionKey = World.makeSubscriptionKey ()
  