You're right there. A developer must implement and/or extend depending on whether it's an abstract or an interface. And that's possibly somewhat of a downfall. Although I can't seem to think of any other way around that, you'll just have to capitalise, underline and embolden the fact that ALL CHILD CLASSES MUST EXTEND THE LOVELY ABSTRACT.

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The man who comes back through the Door in the Wall will never be quite the same as the man who went out.