Goal:
  
  A : Type
  P : nat → A → A → Type
  idx0 : nat
  idx : sigma (λ _ : list A, list A)
  a : OnOne2i P (pr1 {| pr1 := idx0; pr2 := idx |})
        (pr1 (pr2 {| pr1 := idx0; pr2 := idx |}))
        (pr2 (pr2 {| pr1 := idx0; pr2 := idx |}))
  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
        {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |} b
  ============================
   ({| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |} = b)


DEBUG:Going to execute:
fun id =>
             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