Multipart keys

One thing that significantly affects the design of an ID/IDREF replacement
mechanism is whether we support multipart keys (eg the firstName attribute
and lastName attribute together uniquely identify person elements).

Which side of the 80/20 divide do multipart keys lie on?

Note that I am not counting relative or hierarchical keys as multipart keys.
Saying that department names uniquely identify departments within companies
would not count as a multipart key.

James