Yup, that did it (i.e., changing @mode on <valList> to "change") Elisa, thanks! But that begs the question as to what's going on. Even if having "change" as the value is the right way to do this (which, although not obvious, is certainly not silly, either), why did I get three different results with 1) <valList>, 2) <valList mode=add>, and 3) <valList mode=change>? If the default value of @mode is "add" (which is what the documentation for att.combinable says is the default), then (1) & (2) should have been the same. If the default value of @mode here is "change, because that's what my parent had", then (1) and (3) should have been the same.
My first thought is I could swear I just tried to do this exact thing to another attribute list in the past month, and I thought it worked but now I am not so sure... I'll take a look and report back. My second thought is, apparently the positioning of @mode makes a big difference. What happens if you put @mode="change" on valList, leaving everything else the same?
Very creeped out by this ODD behavior,