intros k l; split; intros [y H].
- exists y...
- induction H as [a l eq|a l H IH].
+ destruct a as [k' y'].
exists y'...
+ destruct IH as [e H0].
exists e...
Qed.

Lemma Sort_Inf_NotIn :
forall l k e, Sort l -> Inf (k,e) l ->  ~In k l.