[jononor]

Where does the update of `x` go in this example? By observing a Queue or Stream `q`?

[xapata]

If it must be mutable state, then ideally you'd have a single consumer thread for counting off that queue to avoid worrying about locks.

Or maybe what's in the queue gets written to a permanent log and ``x`` is a query of that log, a la Datomic.