proposed Primer editorial change

There hasn't been a comment about this, but I'd like to add, in Section 
2.3 of the Primer (talking about blank nodes), at the end of the 
paragraph that currently ends "If it is expected that a node in a graph 
will need to be referenced from outside the graph, a URIref should be 
assigned to identify it", the following text:

"Finally, because blank node identifiers represent (blank) nodes, rather 
than arcs, in the triple form of an RDF graph, blank node identifiers 
may only appear as subjects or objects in triples;  blank node 
identifiers may not be used as predicates in triples."

The reason is that this point is not explicitly made in the current Primer.

--Frank