Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  Γ : list A
  Γ' : list A
  a : A
  Δ' : list A
  H' : All2_fold P ([] ++ Γ) ((a :: Δ') ++ Γ')
  H0 : BinInt.Z.of_nat #|Γ| =
       BinInt.Z.add
         (BinInt.Z.add (BinInt.Z.of_nat #|Δ'|) (BinInt.Z.of_nat #|Γ'|))
         (BinNums.Zpos BinNums.xH)
  H : BinInt.Z.of_nat #|Γ| = BinInt.Z.of_nat #|Γ'|
  cstr : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|Δ'|)
  cstr0 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|Γ'|)
  cstr1 : BinInt.Z.le BinNums.Z0 (BinInt.Z.of_nat #|Γ|)
  ============================
   (All2_fold P Γ Γ'
    × All2_fold (λ Δ Δ' : list A, P (Δ ++ Γ) (Δ' ++ Γ')) [] (a :: Δ'))


DEBUG:Going to execute:
idtac