Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  b0 : All2i R n [] []
  b : sigma
        (λ index : sigma (λ _ : nat, sigma (λ _ : list A, list B)),
           All2i R (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  ============================
   ({| pr1 := {| pr1 := n; pr2 := {| pr1 := []; pr2 := [] |} |}; pr2 := b0 |} =
    b
    → let H := CoreTactics.block in
      NoConfusionHom_All2i A B R n [] [] All2i_nil b0
      → let H0 := CoreTactics.block in All2i_nil = 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 index : sigma
                                           (λ _ : nat,
                                              sigma (λ _ : list A, list B)),
                                 All2i R (pr1 index) 
                                   (pr1 (pr2 index)) 
                                   (pr2 (pr2 index))))
Level 4: type of id
 has value 
(sigma
   (λ index : sigma (λ _ : nat, sigma (λ _ : list A, list B)),
      All2i R (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index))))