With canonical S-expressions, unordered sets are a problem because part of the point is to be able to have a single canonical sequence of bytes, which can be ha

With canonical S-expressions, unordered sets are a problem because part of the point is to be able to have a single canonical sequence of bytes, which can be hashed or compared bytewise for equality. In general, I'd resist specifying data as arbitrary key-value pairs, but if I decided that I indeed needed them, I'd do exactly as you suggest — and I'd mandate that the be sorted lexicographically by their keys.