Proof with eauto.
unfold valid_for_reduce.
intuition.
unfold reduce_step.
pose proof (pop_stack_invariant_conserved (prod_rhs_rev prod) stack _ (prod_action' prod)).
destruct (pop (prod_rhs_rev prod) stack (prod_action' prod)) as [|[]].
apply H0...
destruct H0...
pose proof (goto_head_symbs (state_of_stack init s) (prod_lhs prod)).
pose proof (goto_past_state (state_of_stack init s) (prod_lhs prod)).
unfold bind2.
destruct H0.
specialize (H2 _ H3)...
destruct (goto_table (state_of_stack init stack0) (prod_lhs prod)) as [[]|].
constructor.
simpl.
constructor.
eapply prefix_ass...
unfold state_stack_of_stack; simpl; constructor.
intro; destruct (singleton_state_pred x x0); reflexivity.
eapply prefix_pred_ass...
constructor...
constructor...
destruct stack0 as [|[]]...
destruct (compare_eqdec (prod_lhs prod) (start_nt init))...
apply n, H2, eq_refl.
apply H2, eq_refl.
Qed.

(* Specifications: *)
(** If the automaton is safe, then the stack invariant is conserved by [step] and [step] does not crash. **)
Lemma step_stack_invariant_conserved (stack:stack) buffer:
stack_invariant stack ->
match step init stack buffer with
| OK (Fail_sr | Accept_sr _ _) => True
| OK (Progress_sr stack_new _) => stack_invariant stack_new
| Err => False
end.