proposition: Lemma sum_cut_nth n (f: nat -> X A B) i k: n<k -> sum i k f == sum i n f + f (i+n)%nat + sum (i+S n) (k-n-1) f.
proof: Proof. intros; pattern k at 1; replace k with (S(k-n-1)+n)%nat by lia. rewrite sum_cut. rewrite sum_enter_left, plus_assoc. auto with compat lia. Qed.
imports: ATBR(Common) ATBR(Classes) ATBR(Graph)