departureKeyEqualsThePeerKeyThatWasDiscovered

After an arrival and then a departure, departures() emits a key equal to the Tag.peerKey of the Tag discoveries() emitted for that peer.

Asserts its own preconditions rather than assuming them: discoveries() must have produced a Tag (else there is no key to compare against and the property means nothing), and departures() must have produced something (else the comparison is never reached and a silent source passes).

Both feeds stay collected across the departure, deliberately. A discoveries() collection that ended at the first Tag would tear down whatever session the source opened, and this property would then red on a source whose only fault is the next one — conflating the two and leaving departuresEmitsWithNoConcurrentDiscoveriesCollector with nothing of its own to catch.