Goal:
  
  A : Type
  P : A → A → Type
  t : A
  x : A
  l : list A
  l' : list A
  r : P x t
  Hall : All2 P l l'
  IHHall : ∀ n : nat,
             nth_error l' n = Some t
             → ∑ t' : A, nth_error l n = Some t' × P t' t
  ============================
   (Some x = Some x)


DEBUG:Going to execute:
F x2 x1 x0
Level 0: Backtrace:

P is used in hypothesis r.
Level 0: Backtrace:

P is used in hypothesis r.
Level 0: Backtrace:

P is used in hypothesis r.
Level 0: Backtrace:

P is used in hypothesis r.
Level 0: Backtrace:

P is used in hypothesis r.
Level 1: Backtrace:

P is used in hypothesis r.
Level 0: Backtrace:

P is used in hypothesis r.