[XXE] Displaying attribute differently when present or not?

[Bill Fenner]

Many (most) elements in my schema can accept an IDREF attribute named
"anchor".  I currently have it styled like


*::attribute(anchor) {
        attribute-content-left: "[a:";
        attribute-content-middle: text-field(attribute, anchor, columns, 10);
        attribute-content-right: "] ";
        show-attribute: when-added;
        font-size: 80%;
}

I would like to have a small button to add an anchor to an element
when it's missing, something like

content: command-button(icon, icon(plus), command, "putAttribute",
parameters, '[id] anchor')

but I haven't been able to figure out how to use this content if
there's no anchor attribute but the attribute-* styled items when
there is an anchor attribute.

Any ideas?

Thanks,
  Bill