Hint Rewrite split_first_PROP using not_conj_notation : norm1.

Lemma semax_post_flipped' :
forall (R': environ->mpred) Espec {cs: compspecs} (Delta: tycontext) (R P: environ->mpred) c,
@semax cs Espec Delta P c (normal_ret_assert R') ->
ENTAIL Delta, R' |-- R ->
@semax cs Espec Delta P c (normal_ret_assert R).