Ltac inv_safe H :=
inv H;
try solve[match goal with
| H : semantics.at_external _ _ _ = _ |- _ =>
simpl in H; congruence
| H : j_at_external _ _ _ = _ |- _ =>
simpl in H; congruence
| H : semantics.halted _ _ _ |- _ =>
simpl in H; unfold cl_halted in H; contradiction
end].

Lemma semax_loop:
forall Delta Q Q' incr body R,
semax Espec Delta Q body (loop1_ret_assert Q' R) ->
semax Espec Delta Q' incr (loop2_ret_assert Q R) ->
semax Espec Delta Q (Sloop body incr) R.