CoInductive exteq : infseq T -> infseq T -> Prop :=
  exteq_intro :
    forall x s1 s2, exteq s1 s2 -> exteq (Cons x s1) (Cons x s2).CoInductive exteq : infseq T -> infseq T -> Prop :=
  exteq_intro :
    forall x s1 s2, exteq s1 s2 -> exteq (Cons x s1) (Cons x s2).

Lemma exteq_inversion : forall (x1:T) s1 x2 s2, exteq (Cons x1 s1) (Cons x2 s2) -> x1 = x2 /\ exteq s1 s2.