Constraints in a Tamino schema

Do you know anything about constraints for XML elements in Tamino? It seems that we cannot define primary keys or uniqueness for xml elements in Tamino Schema.Is uniqueness supported only at the ino:id level (and therefore only for root elements)?

Looking forward for you reply.
Thank you.

Yes, as of today, no equivalent to the SQL UNIQUE statemenmt exists is Tamino other than that the ino:id is guarenteed to be unique.