(* complement for x *+ n and <= or < *)

Lemma ltr_prod_nat (E1 E2 : nat -> R) (n m : nat) :
(m < n)%N -> (forall i, (m <= i < n)%N -> 0 <= E1 i < E2 i) ->
\prod_(m <= i < n) E1 i < \prod_(m <= i < n) E2 i.