Q: implement finally(). Pass basic tests.
t/31-finally: add tests where the callback returns a plain Future.
Add t/36-finally-cancel.t
Q: then(), resolve(), finally(): call on_cancel() only when the upstream future is not ready yet.
Q: use parent, instead of base.
Q: Add an example of finally() to POD.
Q: POD: mention AnyEvent::Promise.
Release 0.090