[Ur] Triggering JavaScript context switches

[Benjamin Barenblat]

The threads demo* explains that ‘Ur/Web [client code] uses cooperative
multi-threading, not the more common preemptive multi-threading. Only
one thread runs at a time, and only particular function calls can
trigger context switches. In this example, sleep is the only such
function that appears.’ What other functions can trigger context
switches?

* http://impredicative.com/ur/demo/threads.desc.html