Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Q : list A → A → Type
  l : list A
  l' : list A
  ============================
   (All2_fold P l l'
    → All_fold Q l
      → All_fold Q l'
        → All2_fold
            (λ (Γ Γ' : list A) (x y : A), Q Γ x × Q Γ' y × P Γ Γ' x y) l l')


DEBUG:Going to execute:
induction 1; [ constructor |  ] => l r; depelim l;
            depelim ltac:(r); constructor