Ah, I'd forgotten it now mutated [[Prototype]] and not the prototype property's [[Prototype]] (IIRC, earlier drafts had it mutating the latter, i.e., `SubClass.

Ah, I'd forgotten it now mutated [[Prototype]] and not the prototype property's [[Prototype]] (IIRC, earlier drafts had it mutating the latter, i.e., `SubClass.prototype.__proto__`). `super` is indeed new semantics though, so that is definitely true.