End NodeSetForward.
Module NodeSetBackward <: NODE_SET.
Definition t := PHeap.t.
Definition empty := PHeap.empty.
Definition add (n: positive) (s: t) : t := PHeap.insert n s.
Definition pick (s: t) :=
match PHeap.findMin s with
| Some n => Some(n, PHeap.deleteMin s)
| None => None
end.
Definition all_nodes {A: Type} (code: PTree.t A) :=
PTree.fold (fun s pc instr => PHeap.insert pc s) code PHeap.empty.
Definition In := PHeap.In.

Lemma empty_spec:
forall n, ~In n empty.