#209: add local extensibility mechanism
#209: add local extensibility mechanism

an attribute with any name may be used in any element except lb

informaticmonad/EAD3,SAA-SDT/EAD-Revision,SAA-SDT/EAD3,SAA-SDT/EAD3,informaticmonad/EAD3,SAA-SDT/EAD-Revision