Proof with try subst; repeat inv_step; eauto 10 using step.
intros Hst1. revert t2. induction Hst1; intros t2 Ht2...
- destruct (IHHst1 _ H2)... destruct H as (? & [])...
- destruct (IHHst1 _ H2)... destruct H as (? & [])...
Qed.

Lemma confluence s t1 t2 : s ≻* t1 -> s ≻* t2 -> exists u, t1 ≻*u /\ t2 ≻* u.
Definition evaluatesin n s t := (s ≻^n t /\ lam t).