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


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