Inductive plus : state -> cont -> state -> cont -> Prop :=
| plus_left : forall s k s2 k2 s' k',
step s k s2 k2 -> star s2 k2 s' k' -> plus s k s' k'.

Lemma plus_step1 s k s' k' :
plus s k s' k' <-> exists n, stepN (S n) s k s' k'.