[chipdart]

> In such case I think I'd go for an internal-DRYing + copy-on-write approach.

I agree. The primary risk of presented by DRY is tight coupling code which only bears similarities at a surface level. Starting off by explicitly keeping the externa bits separate sounds like a good way to avoid the worst tradeoff.

Nevertheless I still prefer the Write Everything Twice (WET) principle, which means mostly the same thing, but following a clear guideline: postpone all de-duplication efforts until it's either obvious there's shared code (semantics and implementation) in >2 occurrences, and always start by treating separate cases as independent cases.