Disallowing is OK for me, but we may be able to allow it.
Case A: The engine supports the http://bar3impl.com but not
http://bar2impl.com
In this case, the engine should know specific elements/attributes in the
http://bar.com namespace that are relevant to extension
http://bar3impl.com. All other elements/attributes in the namespace
should be for http://bar2imp.com and ignorable.
Case B: The engine supports neither.
In this case, the engine may erroneously reject a process definition
that contains elements/attributes for http://bar2impl.com only and does
not contain those for http://bar3imp.com because the engine cannot
distinguish elements/attributes for bar2impl from those for bar3impl.
But I don't think this error severe. Just removing the extension
declaration for bar3impl should do.
Could you elaborate on what you are worrying about?
