Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2 P tl tl'
  hd0 : A
  hd' : A
  tl0 : list A
  p : P hd0 hd'
  ============================
   (NoConfusion_OnOne2 A P
      {|
        pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
        pr2 := OnOne2_tl P hd tl tl' a
      |}
      {|
        pr1 := {| pr1 := hd0 :: tl0; pr2 := hd' :: tl0 |};
        pr2 := OnOne2_hd P hd0 hd' tl0 p
      |}
    → let H := CoreTactics.block in
      {|
        pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
        pr2 := OnOne2_tl P hd tl tl' a
      |} =
      {|
        pr1 := {| pr1 := hd0 :: tl0; pr2 := hd' :: tl0 |};
        pr2 := OnOne2_hd P hd0 hd' tl0 p
      |})


DEBUG:Going to execute:
CoreTactics.intros_until_block
DEBUG LTAC Evaluated term: (NoConfusion_OnOne2 A P
                              {|
                                pr1 :=
                                  {| pr1 := hd :: tl; pr2 := hd :: tl' |};
                                pr2 := OnOne2_tl P hd tl tl' a
                              |}
                              {|
                                pr1 :=
                                  {| pr1 := hd0 :: tl0; pr2 := hd' :: tl0 |};
                                pr2 := OnOne2_hd P hd0 hd' tl0 p
                              |}
                            → let H := CoreTactics.block in
                              {|
                                pr1 :=
                                  {| pr1 := hd :: tl; pr2 := hd :: tl' |};
                                pr2 := OnOne2_tl P hd tl tl' a
                              |} =
                              {|
                                pr1 :=
                                  {| pr1 := hd0 :: tl0; pr2 := hd' :: tl0 |};
                                pr2 := OnOne2_hd P hd0 hd' tl0 p
                              |})