Goal:
  
  A : Type
  P : nat → A → A → Type
  i : nat
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2i P (S i) tl tl'
  i0 : nat
  hd0 : A
  hd' : A
  tl0 : list A
  p : P i0 hd0 hd'
  H : NoConfusion_OnOne2i A P
        {|
          pr1 :=
            {| pr1 := i; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
          pr2 := OnOne2i_tl P i hd tl tl' a
        |}
        {|
          pr1 :=
            {|
              pr1 := i0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd' :: tl0 |}
            |};
          pr2 := OnOne2i_hd P i0 hd0 hd' tl0 p
        |}
  ============================
   (let H := CoreTactics.block in
    {|
      pr1 := {| pr1 := i; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
      pr2 := OnOne2i_tl P i hd tl tl' a
    |} =
    {|
      pr1 :=
        {| pr1 := i0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd' :: tl0 |} |};
      pr2 := OnOne2i_hd P i0 hd0 hd' tl0 p
    |})


DEBUG:Going to execute:
CoreTactics.intros_until_block