Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : R n x y
  a : All2i R (S n) l r
  b0 : All2i R n (x :: l) (y :: r)
  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 := x :: l; pr2 := y :: r |} |};
      pr2 := b0
    |} = b
    → let H := CoreTactics.block in
      NoConfusionHom_All2i A B R n (x :: l) (y :: r) (All2i_cons r0 a) b0
      → let H0 := CoreTactics.block in All2i_cons r0 a = b0)


DEBUG:Going to execute:
hnf in id
Level 5: 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
constr
id := b
of type
constr of type tacvalue

Level 4: 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
constr
id := b
of type
constr of type tacvalue