[toast0]

Making X.upper().lower() return X.lower() is doable with an expanded string type that keeps track of more context, such that modifiers don't apply until the final output. In that case, it would be relatively simple to say that with multiple upper and lower calls, only the last survives.

Making 'STRASSE'.lower() return 'straße' requires that the caller have knowledge of the written language in us and a lookup through a language dictionary. IIRC, not all German words with two consecutive 's' es are properly written with an 'ß', and I don't know much about other languages that use that character. Blindly changing any SS to ß on lowercasing isn't what anyone wants, but rarely are strings annotated with the written language they contain, and it gets worse because strings can contain multiple written languages, which is only extremely rarely annotated.