I understood your point; that's why I used an absolute inclusion in my example (C includes D.  D is NOT a plus card, it is NOT a child, but it IS a subcard).  You're using "subcards" to mean "relative plus cards".  I'm explicitly not.   We should never use that word that way, and to my knowledge, we never do in code.

Absolute inclusions are still fully supported as subcards (though you can't use the syntactic sugar with them).