Remark modarith_idem: forall nv, modarith (modarith nv) = modarith nv.

Lemma mul_sound:
forall v1 w1 v2 w2 x,
vagree v1 w1 (modarith x) -> vagree v2 w2 (modarith x) ->
vagree (Val.mul v1 v2) (Val.mul w1 w2) x.