Definition remove_at {A} i (l : list A) := firstn i l ++ skipn (S i) l.

Lemma iter_sepcon_sepcon: forall {A} f g1 g2 l, (forall b : A, f b = g1 b * g2 b) -> iter_sepcon f l = iter_sepcon g1 l * iter_sepcon g2 l.