Goal:
  
  A : Type
  B : Type
  n : nat
  t : A
  P : B → A → A → Type
  b : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b hd hd'
  e : #|bs| = #|tl|
  hnth : nth_error tl n = Some t
  ============================
   (nth_error tl n = Some t
    × (t = t) + (∑ i' : B, nth_error bs n = Some i' × P i' t t))


DEBUG:Going to execute:
match goal with
           | |- ?X1 → ?X2 => intro
           | |- ¬ _ => unfold not at 1; intro
           end