RDF-DAML question: missing object

[Robert Mark Bram]

Hi All,

I thought that an RDF triple should always include "subject, verb and
object".

But then I found this example about DAML that seems to miss the object..

==
http://infomesh.net/2001/swintro/

The second useful DAML construct that we shall go through is the
daml:UnambiguousProperty class. Saying that a Property is a
daml:UnambiguousProperty means that if the object of the property is the
same, then the subjects are equivalent. For example:-

foaf:mbox rdf:type daml:UnambiguousProperty .
:x foaf:mbox .
:y foaf:mbox .
==

How are these two allowed to miss the object?

Any advice would be most welcome!

Rob