I'm using Stack in a small project; like its simplicity, and thought I'd contribute the tests.
This covers every feature of stack (every statement, branch and function, according to "istanbul" code coverage), and can also serve as an example for general usage as well as for the undocumented Stack.compose and Stack.mount.
Great to see these. You might just use tap instead of tape since stack doesn't get used on the browser (AFAIK), but that's not a blocker. I haven't used compose or mount for anything, but good to know more about them as well.
@gsf good point. I removed it.
add unit-tests (100% coverage)