Goal:
  
  A : Type
  t : A
  P : nat → A → A → Type
  i : nat
  hd' : A
  tl : list A
  p : P i t hd'
  ============================
   (∑ t' : A, Some hd' = Some t' × (t = t') + P (i + 0)%nat t t')


DEBUG:Going to execute:
exists hd'; rewrite Nat.add_0_r