Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  Γ' : list A
  Δ : list A
  Δ' : list A
  H : #|Γ| = #|Γ'|
  ============================
   (All2_fold P (Δ ++ Γ) (Δ' ++ Γ')
    → All2_fold P Γ Γ'
      × All2_fold (λ Δ Δ' : list A, P (Δ ++ Γ) (Δ' ++ Γ')) Δ Δ')


DEBUG:Going to execute:
induction Δ in H, Δ', Γ, Γ' |- *; destruct Δ'