
(calling `Instances.set_reachable/1` only if instance had `unreachable_since`, calling `Instances.set_unreachable/1` only if instance had nil `unreachable_since`).
(calling `Instances.set_reachable/1` only if instance had `unreachable_since`, calling `Instances.set_unreachable/1` only if instance had nil `unreachable_since`).