anchor.diff:              (subscriptionAddress'', unsubscribe, world)
           { ObserverAddress = stream.ObserverAddress; Subscribe = subscribe }
   
  -    /// Filter event stream by the 'pred' procedure.
  +    /// 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 ()
  
positive.code:             (subscriptionAddress, unsubscribe, world)
          { ObserverAddress = stream.ObserverAddress; Subscribe = subscribe }
  
      let track2
          (tracker : 'a -> Event<'a, 'o> -> World -> 'a * bool)
          (stream : EventStream<'a, 'o>) :
  
negative.code:             oldValue <> newValue)
              stream
  
      /// Filter out simulant change events that do not relate to those returned by 'valueGetter'.
      let simulantValue (valueGetter : 'a -> 'b) (stream : EventStream<'a SimulantChangeData, 'o>) =
          filter (fun a w ->
              let oldValue = valueGetter a.Data.OldSimulant
  