On: Inheritance was invented as a performance hack

[toomanybeersies]

Breaking the implementation of CountingStack by changing the implementation of Stack is the kind of stuff that unit tests should pick up. It's easy to write a test case for "CountingStack.pushAll() counts once per given element"

[aflag → toomanybeersies]

Tests are not bullet proof and subtle errors may go uncatched. If you can completely eliminate a source of errors, why not?