Is it (still?) possible to apply no structure/default (i.e., go back to being a Basic card) for a card of type foo if foo+*type+*structure exists? (And if so, how?)
set content of self structure card to "_self"
clarification: it doesn't have to be a +*self set for this to work. Any time the *structure rule has that content "_self", that's equivalent to having no structure rule.