As Lukasz mentions in NOTES-ambv.txt: "The covariance/contravariance discussion needs to happen at some point."

Reference: http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)

I haven't fully digested all the viewpoints yet. It seems that mypy stays on the safe side and treats e.g. Set[Employee] and Set[Manager] as distinct types. However, Set[Employee] is acceptable where a Set (without parameter) is expected.