Goal:
  
  A : Type
  P : list A → A → Type
  Q : list A → list A → A → A → Type
  Γ' : list A
  d : A
  Γ : list A
  X : All_fold P Γ
  p : P Γ d
  IHX : ∀ Γ' : list A,
          All2_fold Q Γ Γ'
          → All2_fold (λ (Γ Γ'0 : list A) (d d' : A), P Γ d × Q Γ Γ'0 d d') Γ
              Γ'
  H1 : sigma (λ _ : list A, list A)
  H : All2_fold Q (pr1 H1) (pr2 H1)
  ============================
   ({| pr1 := d :: Γ; pr2 := Γ' |} = H1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2_fold (λ (Γ Γ' : list A) (d d' : A), P Γ d × Q Γ Γ' d d') 
        (d :: Γ) Γ')


DEBUG:Going to execute:
fun id id' =>
             hnf in id;
              match type of id with
              | sigma _ =>
                  let xn := fresh id "'" in
                  destruct id as [xn id]; DepElim.decompose_exists xn id;
                   cbv beta iota delta [TODO projection TODO projection]
                    in id, id'; DepElim.decompose_exists id id'
              | _ =>
                  cbv beta iota delta [TODO projection TODO projection]
                   in id, id'
              end