A database holding a fan-out over {:upper, "a"}, {:upper, "b"} or
both, as a run left it, and the input of "a" changed since: the
group is stale. Written directly rather than computed, to keep the
scenarios Concuerror explores short (test/concurrency/group_test.ex).
Summary
Functions
Deletes the fixture's tables.
The fan-out's body: {:upper, key} for each of the keys it is keyed by.
A database whose fan-out over members ("a", "b" or both), keyed
by them, went stale when "a" changed.