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:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]