(header ((client) hello_optional_nondet provides) ((sut) hello_optional_nondet interface))
<external>.hello -> ...
... -> sut.hello
... <- sut.false
<external>.false <- ...
(trail "hello" "false")
... <- sut.world
<external>.world <- ...
(trail "world")
(labels "hello" "optional" "inevitable")
(eligible "inevitable" "optional")
