Goal:
  
  A : Type
  P : nat → A → A → Type
  i : nat
  hd : A
  hd' : A
  tl : list A
  p : P i hd hd'
  i0 : nat
  hd0 : A
  hd'0 : A
  tl0 : list A
  p0 : P i0 hd0 hd'0
  ============================
   (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 := i0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd'0 :: tl0 |} |};
        pr2 := OnOne2i_hd P i0 hd0 hd'0 tl0 p0
      |}
    → 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 := i0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd'0 :: tl0 |} |};
        pr2 := OnOne2i_hd P i0 hd0 hd'0 tl0 p0
      |})


DEBUG:Going to execute:
CoreTactics.intros_until_block
DEBUG LTAC Evaluated term: (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 := i0;
                                    pr2 :=
                                      {|
                                        pr1 := hd0 :: tl0; pr2 := hd'0 :: tl0
                                      |}
                                  |};
                                pr2 := OnOne2i_hd P i0 hd0 hd'0 tl0 p0
                              |}
                            → 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 := i0;
                                    pr2 :=
                                      {|
                                        pr1 := hd0 :: tl0; pr2 := hd'0 :: tl0
                                      |}
                                  |};
                                pr2 := OnOne2i_hd P i0 hd0 hd'0 tl0 p0
                              |})