Definition thread_step_plus U c m c' m' :=
exists n, thread_stepN (S n) U c m c' m'.
Definition thread_step_star U c m c' m' :=
exists n, thread_stepN n U c m c' m'.

Lemma thread_step_star_trans: forall U c1 c2 c3 m1 m2 m3,
thread_step_star U c1 m1 c2 m2 -> thread_step_star U c2 m2 c3 m3 ->
thread_step_star U c1 m1 c3 m3.