Things that are coincidentially similar are much "larger" and more trivial than something that becomes easier and simpler if it is more abstract like a HMAC alg

Things that are coincidentially similar are much "larger" and more trivial than something that becomes easier and simpler if it is more abstract like a HMAC algorithm; for example accounting applications would care about records that need VAT and records that reference a counterpart, which are approximately 90% the same and the other 90% special cases with business logic organized according to personal taste.