Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  x' : B
  y : B
  l' : list B
  H : y = x' ∨ In x' l'
  x : A
  l : list A
  r : P x y
  X : All2 P l l'
  IHX : In x' l' → ∥ ∑ x : A, P x x' ∥
  ============================
   (∑ x : A, P x x')


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.