Inductive stack_approx_rel (n:nat) : stack -> stack -> Prop :=
| stack_approx_nil : stack_approx_rel n nil nil
| stack_approx_cons : forall i i' stk stk',
instr_approx_rel n i i' ->
stack_approx_rel n stk stk' ->
stack_approx_rel n (i::stk) (i'::stk').

Lemma step_prog: forall p1 p2 r1 s1 r2 s2,
step p1 p2 r1 s1 r2 s2 ->
p1 = p2 \/ age p1 p2.