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For what it's worth, I think that there is a worthwhile concept in a "unicode corresponding character subset." That is, within each implementation, the set of characters which can be represented by single corresponding codepoints of unicode. This won't be the same set in every scheme. You can define your FFI's semantics on what happens when strings are composed entirely of the unicode-correspondent set, leave un(der)specified what happens when the c side returns a unicode character unknwon to the scheme system or when the scheme system uses a character unknown to unicode, and let the implementors worry about their implementations' extensions. There should probably be predicates to ask whether particular entities (characters or codepoint-numbers) are part of the unicode corresponding character subset for the given scheme. Bear