(header ((h) ihello provides) ((sut) async_provides component))
<external>.h.hello -> ...
... -> sut.h.hello
... <- sut.h.return
<external>.h.return <- ...
(trail "h.hello" "h.return")
... <- sut.h.world
<external>.h.world <- ...
(trail "h.world")
(labels "h.hello")
(eligible "h.hello")
