And yet somehow I'm confident that this will actually run properly the first time...

gistfile1.fs

    let product address (observable : 'a Observable) : ('a * 'b) Observable =
        let subscribe = fun world ->
            let subscriptionKey = World.makeSubscriptionKey ()
            let subscriptionKey' = World.makeSubscriptionKey ()
            let subscriptionAddress = !+ [acstring subscriptionKey]
            let subscriptionAddress' = !+ [acstring subscriptionKey']
            let (address', unsubscribe, world) = observable.Subscribe world
            let unsubscribe = fun world ->
                let world = unsubscribe world
                let world = World.unsubscribe subscriptionKey world
                World.unsubscribe subscriptionKey' world
            let subscription = fun event world ->
                let subscription' = fun event' world ->
                    let eventData = (event.Data, event'.Data)
                    let world = World.publish<'a * 'b> World.sortSubscriptionsNone subscriptionAddress subscriptionAddress eventData world
                    (Cascade, world)
                let world = World.subscribe<'b> subscriptionKey' address' subscriptionAddress' subscription' world
                (Cascade, world)
            let world = World.subscribe<'a> subscriptionKey address subscriptionAddress subscription world
            (subscriptionAddress, unsubscribe, world)
        { Subscribe = subscribe; TypeCarrier = fun (_ : 'a * 'b) -> () }