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


DEBUG:Going to execute:
<coq-core.plugins.ltac::exists@1> $1