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'
  ============================
   (∀ (H := CoreTactics.block) (e : 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 H0 := CoreTactics.block in
      noConfusion_OnOne2_obligation_2 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
        |}
        (noConfusion_OnOne2_obligation_1 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
           |} e) = e)


DEBUG:Going to execute:
match T with
           | context [ CoreTactics.block ] =>
               change T; CoreTactics.intros_until_block
           end