Mutually exclusive content type required

 WebDAV has a requirement that properties should be unique. In other words, they don't care what the element name is, as long as there is only one element with that name. http://andrew2.andrew.cmu.edu/rfc/rfc2518.html  s4.5

This is an interesting case because it is the same requirement as that of IDs: that they are unique: the scope is perhaps different  (within an element versus within a document) but the requirement seems similar.

Can we have a restricted form of ANY to cope with this? Perhaps "ENUM" might be consistant.

Rick Jelliffe