Goal:
  
  A : Type
  P : list A → A → Type
  a : A
  Γ : list A
  Δ : list A
  IHΓ : ∀ Δ : list A,
          All_fold P (Γ ++ Δ)
          → All_fold P Δ × All_fold (λ Γ : list A, P (Γ ++ Δ)) Γ
  X1 : list A
  X : All_fold P X1
  ============================
   (a :: Γ ++ Δ = X1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All_fold P Δ)


DEBUG:Going to execute:
try rename id into id',id' into id
Level 3: evaluation returns
try rename id into id',id' into id; cont id'1 id
where
t := a :: Γ ++ Δ
of type
constr
id'1 := X1
of type
constr
id' := X
of type
constr
id := X
of type
constr
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := X
        of type
        constr
of type
tacvalue of type tacvalue