Goal:
  
  A : Type
  P : A → A → Type
  a : sigma
        (λ index : sigma (λ _ : list A, list A),
           OnOne2 P (pr1 index) (pr2 index))
  b : sigma
        (λ index : sigma (λ _ : list A, list A),
           OnOne2 P (pr1 index) (pr2 index))
  H : NoConfusion_OnOne2 A P a b
  ============================
   (a = b)


DEBUG:Going to execute:
CoreTactics.destruct_sigma a; CoreTactics.destruct_sigma b;
            destruct a; DepElim.depelim b; simpl in * |-;
            CoreTactics.on_last_hyp
             ltac:(fun id =>
                     hnf in id;
                      CoreTactics.destruct_tele_eq id || destruct id)