Fixpoint effstepN (n:nat) : (block -> Z -> bool) -> C -> mem -> C -> mem -> Prop :=
    match n with
      | O => fun U c m c' m' => (c,m) = (c',m')
      | S k => fun U c1 m1 c3 m3 => exists c2, exists m2,
        effstep Sem g U c1 m1 c2 m2 /\
        effstepN k (fun b ofs => U b ofs  || freshloc m1 m2 b)  c2 m2 c3 m3
    end.

Lemma effstepN_sub: forall n U V c m c' m' (UV: forall b ofs, U b ofs = true -> V b ofs = true), (effstepN n U c m c' m' -> effstepN n V c m c' m').