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_seq:
forall Delta (R: ret_assert) P Q h t,
semax Espec Delta P h (overridePost Q R) ->
semax Espec Delta Q t R ->
semax Espec Delta P (Clight.Ssequence h t) R.