Goal:
  
  A : Type
  P : nat → A → Type
  n : nat
  x : A
  Γ : list A
  IHΓ : All_fold (λ (Γ : list A) (d : A), P (n + #|Γ|) d) (List.rev Γ)
        → Alli P n Γ
  a1 : list A
  a : All_fold (λ (Γ : list A) (d : A), P (n + #|Γ|) d) a1
  ============================
   (x :: List.rev Γ = a1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in Alli P n (Γ ++ [x]))


DEBUG:Going to execute:
DepElim.decompose_exists id id'