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


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