Foreign superkey support

Are there any DMBSs apart from Dataphor that support foreign superkeys without undue hassle? SQL systems support them in that a superkey may be declared UNIQUE, and thus referred to by a foreign key, but I'm looking for more explicit support.

Or more generally, are there any DBMSs that support inclusion dependencies (apart from the special case of foreign keys, naturally) directly?

Or for that matter, how widespread is multi-relvar constraint support (apart from foreign keys)? As far as I know, neither Oracle nor MSSQL support SQL ASSERTIONs. Are there any DMBSs that do?

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Jon