error in Semantics document

The following claim from RDF Semantics, Section 0.3, is incorrect:

	In particular, any graph obtained by replacing all blank nodes by
	new blank nodes not in the original graph is an instance of the
	original and also, by inverting the mapping, has it as an
	instance. By our convention, such isomorphic graphs are considered
	to be identical.

To see this consider the ``original'' graph

	a:b c:d _:a .
	a:b c:e _:b .

and the ``new'' graph

	a:b c:d _:c .
	a:b c:e _:c .

peter