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


DEBUG:Going to execute:
fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]