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 (pr1 {| pr1 := b''; pr2 := b' |})
        (pr1 (pr2 {| pr1 := b''; pr2 := b' |}))
        (pr2 (pr2 {| pr1 := b''; pr2 := b' |}))
  ============================
   ({| pr1 := {| pr1 := n; pr2 := {| pr1 := []; pr2 := [] |} |}; pr2 := b0 |} =
    {| pr1 := {| pr1 := b''; pr2 := b' |}; pr2 := b |}
    → ∀ (H := CoreTactics.block) (e : NoConfusionHom_All2i A B R n [] []
                                        All2i_nil b0),
        let H0 := CoreTactics.block in
        noConfusionHom_All2i_obligation_2 A B R n [] [] All2i_nil b0
          (noConfusionHom_All2i_obligation_1 A B R n [] [] All2i_nil b0 e) =
        e)


DEBUG:Going to execute:
hnf in id
Level 6: evaluation returns
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
where
id' := b'
of type
intropattern
id := b''
of type
intropattern of type tacvalue