Proof NodeSetForward.add_spec.

(* Specifications: *)
Lemma pick_none:
forall s n, pick s = None -> ~In n s.
Lemma pick_some:
forall s n s', pick s = Some(n, s') ->
forall n', In n' s <-> n = n' \/ In n' s'.
Lemma all_nodes_spec:
forall A (code: PTree.t A) n instr,
code!n = Some instr -> In n (all_nodes code).