(** addition and modulo  Generally speaking, unlike with other conventions, we don't have [(a+b) rem n = (a rem n + b rem n) rem n] for any a and b. For instance, take (8 + (-10)) rem 3 = -2 whereas (8 rem 3 + (-10 rem 3)) rem 3 = 1. *)

Lemma add_rem_idemp_l : forall a b n, n~=0 -> 0 <= a*b ->
((a rem n)+b) rem n == (a+b) rem n.