private static async Task Execute(
InMemoryEventStore store,
string streamName,
Func <EventData[], IEvent[]> f,
Func <IEvent[], Task> pub)
{
var r = await store.LoadEventStreamAsync(streamName, 0);
var events = f(r.Events.ToArray());
_ = await store.AppendToStreamAsync(streamName, r.Version, events);
await pub(events);
}