Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  b0 : All2i R n [] []
  b'' : nat
  b' : sigma (λ _ : list A, list B)
  b : All2i R b'' (pr1 b') (pr2 b')
  ============================
   ({| pr1 := {| pr1 := n; pr2 := {| pr1 := []; pr2 := [] |} |}; pr2 := b0 |} =
    {| pr1 := {| pr1 := b''; pr2 := b' |}; pr2 := 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:
PTRDEBUGTAClet 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, list B))
Level 6: type of id
 has value 
(sigma (λ _ : list A, list B))