Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  idx2 : list A
  idx1 : list A
  b : All2_fold P idx2 idx1
  e : NoConfusion_All2_fold A P
        {| pr1 := {| pr1 := []; pr2 := [] |}; pr2 := All2_fold_nil P |}
        {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |}
  ============================
   (let H := 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:
CoreTactics.revert_until H
DEBUG LTAC Evaluated term: (let H := 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)