Definition effstep_plus U c m c' m' :=
    exists n, effstepN (S n) U c m c' m'.Definition effstep_star U c m c' m' :=
    exists n, effstepN n U c m c' m'.

Lemma effstep_plus_valid U c1 m1 c2 m2 (Step: effstep_plus U c1 m1 c2 m2): forall b z, U b z = true -> Mem.valid_block m1 b.