Tangential: PDA aren’t really the “next-lower” model, just the one that we talk about most often; there’s a whole family of models that are stronger than a PDA

Tangential: PDA aren’t really the “next-lower” model, just the one that we talk about most often; there’s a whole family of models that are stronger than a PDA but not Turing-complete (e.g. alternating pushdown automata).