Goal:
    A : Type  t : A  P : A → A → Type  hd' : A  tl : list A  p : P t hd'
  ============================
   (Some hd' = Some hd' × (t = hd') + P t hd')


DEBUG:Going to execute:
<Coq.Init.Tauto.is_disj> flags X1; elim id; intro