Proof with norm.
intros.
apply @derives_trans with (P * emp).
rewrite sepcon_emp...
apply sepcon_derives...
Qed.
Hint Resolve @sepcon_TT.

Lemma imp_extract_exp_left {B A: Type} {NA: NatDed A}:
forall    (p : B -> A) (q: A),
(forall x, p x |-- q) ->
exp p |-- q.