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


DEBUG:Going to execute:
<coq-core.plugins.ltac::exists@1> $1
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

Tactic failure: congruence failed.