Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  idx2 : list A
  idx1 : list A
  b : All2_fold P idx2 idx1
  ============================
   (∀ (H := CoreTactics.block) (e : NoConfusion_All2_fold A P
                                      {|
                                        pr1 := {| pr1 := []; pr2 := [] |};
                                        pr2 := All2_fold_nil P
                                      |}
                                      {|
                                        pr1 := {| pr1 := idx2; pr2 := idx1 |};
                                        pr2 := b
                                      |}),
      let H0 := CoreTactics.block in
      noConfusion_All2_fold_obligation_2 A P
        {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |}
        {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |}
        (noConfusion_All2_fold_obligation_1 A P
           {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |}
           {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |} e) = e)


DEBUG:Going to execute:
fun p => DepElim.elim_tac ltac:(fun p el => destruct p using el) p