Goal:
  
  A : Type
  P : nat → A → A → Type
  i : nat
  hd : A
  hd' : A
  tl : list A
  p : P i hd hd'
  idx3 : nat
  idx4 : list A
  idx2 : list A
  b : OnOne2i P idx3 idx4 idx2
  H : NoConfusion_OnOne2i A P
        {|
          pr1 :=
            {| pr1 := i; pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |} |};
          pr2 := OnOne2i_hd P i hd hd' tl p
        |}
        {|
          pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
          pr2 := b
        |}
  ============================
   (let H := CoreTactics.block in
    {|
      pr1 := {| pr1 := i; pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |} |};
      pr2 := OnOne2i_hd P i hd hd' tl p
    |} =
    {|
      pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
      pr2 := b
    |})


DEBUG:Going to execute:
CoreTactics.revert_until H
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            {|
                              pr1 :=
                                {|
                                  pr1 := i;
                                  pr2 :=
                                    {| pr1 := hd :: tl; pr2 := hd' :: tl |}
                                |};
                              pr2 := OnOne2i_hd P i hd hd' tl p
                            |} =
                            {|
                              pr1 :=
                                {|
                                  pr1 := idx3;
                                  pr2 := {| pr1 := idx4; pr2 := idx2 |}
                                |};
                              pr2 := b
                            |})