Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → B → C → Type
  x : A
  y : B
  z : C
  l : list A
  l' : list B
  l'' : list C
  r : R x y z
  a : All3 R l l' l''
  b0 : All3 R (x :: l) (y :: l') (z :: l'')
  b' : sigma (λ _ : list A, sigma (λ _ : list B, list C))
  b : All3 R (pr1 b') (pr1 (pr2 b')) (pr2 (pr2 b'))
  ============================
   ({|
      pr1 :=
        {| pr1 := x :: l; pr2 := {| pr1 := y :: l'; pr2 := z :: l'' |} |};
      pr2 := b0
    |} = {| pr1 := b'; pr2 := b |}
    → let H := CoreTactics.block in
      NoConfusionHom_All3 A B C R (x :: l) (y :: l') 
        (z :: l'') (All3_cons r a) b0
      → let H0 := CoreTactics.block in All3_cons r a = b0)


DEBUG:Going to execute:
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'
DEBUG LTAC Evaluated term: b'
DEBUG LTAC Evaluated term: (sigma
                              (λ _ : list A, sigma (λ _ : list B, list C)))
Level 5: type of id
 has value 
(sigma (λ _ : list A, sigma (λ _ : list B, list C)))