On: A Cross-Platform String Library Written in Swift

[TazeTSchnitzel]

None. But letters can be neither lowercase nor uppercase.

[setr → TazeTSchnitzel]

In which case, the function(s) should really be throwing some kind of error, since they can't reasonably be applied to the input.

But checking !(isUppercase(str) || isLowercase(str)) to see if its not case-able(?) is absurd

Obviously you'd instead want the function notCaseable(str).