Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  idx : sigma (λ _ : list A, list A)
  a : All2_fold P (pr1 idx) (pr2 idx)
  b : sigma
        (λ index : sigma (λ _ : list A, list A),
           All2_fold P (pr1 index) (pr2 index))
  e : NoConfusion_All2_fold A P {| pr1 := idx; pr2 := a |} b
  ============================
   (noConfusion_All2_fold_obligation_2 A P {| pr1 := idx; pr2 := a |} b
      (noConfusion_All2_fold_obligation_1 A P {| pr1 := idx; pr2 := a |} b e) =
    e)


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