Goal:
  
  A : Type
  P : nat → A → A → Type
  a : sigma
        (λ index : sigma (λ _ : nat, sigma (λ _ : list A, list A)),
           OnOne2i P (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  b : sigma
        (λ index : sigma (λ _ : nat, sigma (λ _ : list A, list A)),
           OnOne2i P (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  H : NoConfusion_OnOne2i A P a b
  ============================
   (a = b)


DEBUG:Going to execute:
match type of id with
           | sigma ?P =>
               let idx := fresh "idx" in
               destruct id as [idx id]; repeat CoreTactics.destruct_sigma idx;
                simpl in id
           | _ => idtac
           end