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
  a : All_fold (λ (Γ : list A) (d : A), P (n + #|Γ|) d) (x :: List.rev Γ)
  a0 := {| pr1 := x :: List.rev Γ; pr2 := a |}
     : sigma
         (λ index : list A,
            All_fold (λ (Γ : list A) (d : A), P (n + #|Γ|) d) index)
  a1 := x :: List.rev Γ : list A
  a2 := a
     : (λ index : list A,
          All_fold (λ (Γ : list A) (d : A), P (n + #|Γ|) d) index) a1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in Alli P n (Γ ++ [x]))


DEBUG:Going to execute:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2; hnf in id'2; 
            simpl in id'2; rename id'2 into id; cont id id id'1 t