Proof with simpl; auto.
intros; unfold normalize_atoms; rewrite listd_sort_uniq...
induction l... rewrite <-order_eqv_sound, <-IHl...
Qed.

Lemma list_denote_union_normalize_pure_atoms B l :
list_denote pure_atom_denote un B l =
list_denote pure_atom_denote un B (normalize_atoms l).