I want my customization to add a few possible values to the list P5 provides for a semi-closed attribute list. E.g., the @type of <idno>. P5 says it has "ISBN", "ISSN", "DOI", "URI", "VIAF", "ESTC", and "OCLC". We'd like our customization to add "LC" and "LCCN" to that list. None of the expected combinations of values of @mode ("add" and "change") that I've tried have worked. Two of the more obvious combinations: --------- A --------- <elementSpec module="header" ident="idno" mode="change"> <gloss>JUST A TEST GLOSS to make sure <val>change</val> is working</gloss> <attList> <attDef ident="type" mode="change"> <valList type="semi"> <valItem ident="LC" mode="add"> <gloss>U.S. Library of Congress call number</gloss> </valItem> <valItem ident="LCCN" mode="add"> <gloss>Library of Congress control number</gloss> </valItem> </valList> </attDef> </attList> </elementSpec> --------- B --------- Same, but added @mode of "add" to <valList> For (A), nothing was added -- the list I ended up with was the same as P5. For (B) the 2 new values replaced the existing ones, so I had only those, and none of the 7 from P5. Any thoughts?