Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  idx0 : list A
  idx : list A
  a : All2_fold P idx0 idx
  idx1 : sigma (λ _ : list A, list A)
  b : All2_fold P (pr1 idx1) (pr2 idx1)
  H : NoConfusion_All2_fold A P
        {| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |}
        {| pr1 := idx1; pr2 := b |}
  ============================
   ({| pr1 := {| pr1 := idx0; pr2 := idx |}; pr2 := a |} =
    {| pr1 := idx1; pr2 := 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