Unicode libraries commonly provide functions that take a code point and return a value, for example a property value. Such a function normally accepts the whole range 0..10ffff (and may even return a default value for out-of-range inputs).
Also, we commonly read code points from 16-bit Unicode strings, and unpaired surrogates are returned as themselves and treated as such (e.g., in collation). That would not be well-formed UTF-16, but it's generally harmless in text processing. markus
