Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2 P tl tl'
  idx2 : list A
  idx1 : list A
  b : OnOne2 P idx2 idx1
  H : NoConfusion_OnOne2 A P
        {|
          pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
          pr2 := OnOne2_tl P hd tl tl' a
        |} {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |}
  ============================
   ({|
      pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
      pr2 := OnOne2_tl P hd tl tl' a
    |} = {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |})


DEBUG:Going to execute:
intro_block_id H
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            NoConfusion_OnOne2 A P
                              {|
                                pr1 :=
                                  {| pr1 := hd :: tl; pr2 := hd' :: tl |};
                                pr2 := OnOne2_hd P hd hd' tl p
                              |}
                              {|
                                pr1 := {| pr1 := idx2; pr2 := idx1 |};
                                pr2 := b
                              |}
                            → let H0 := CoreTactics.block in
                              {|
                                pr1 :=
                                  {| pr1 := hd :: tl; pr2 := hd' :: tl |};
                                pr2 := OnOne2_hd P hd hd' tl p
                              |} =
                              {|
                                pr1 := {| pr1 := idx2; pr2 := idx1 |};
                                pr2 := b
                              |})