about reification and exactly one

In http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/#model
5. Formal Model for RDF
...
When a resource represents a reified statement; that is,
it has an RDF:type property with a value of RDF:Statement,
then that resource must have exactly one RDF:subject property,
one RDF:object property, and one RDF:predicate property.
...
So exactly one, not zero, not more than one.
I think this is not difficult at all.
The things to model a statement are clearly defined
and agents must be made consistent in this respect.

Jonathan, there is indeed nothing in the spec to stop
you doing reification of reification statements, so I
was wrong. The "4 exponent reificationlevel" size issue
is not relevant for our internal model representation.

Jos De Roo -- AGFA