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

Lemma TT_sepcon {A} {NA: NatDed A}{SA: SepLog A}{CA: ClassicalSep A}:
forall (P: A), P |-- (TT * P).