Goal:
  
  A : Type
  P : list A → A → Type
  d : A
  Γ : list A
  a : All_fold P Γ
  p : P Γ d
  b0 : All_fold P (d :: Γ)
  b : sigma (λ index : list A, All_fold P index)
  ============================
   ({| pr1 := d :: Γ; pr2 := b0 |} = b
    → let H := CoreTactics.block in
      NoConfusionHom_All_fold A P (d :: Γ) (All_fold_cons P a p) b0
      → let H0 := CoreTactics.block in All_fold_cons P a p = b0)


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