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


DEBUG:Going to execute:
cbn