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
  tl0 : list A
  tl' : list A
  b : OnOne2i P (S i0) tl0 tl'
  H : 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 := hd0 :: tl' |}
            |};
          pr2 := OnOne2i_tl P i0 hd0 tl0 tl' b
        |}
  ============================
   (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 := hd0 :: tl' |} |};
      pr2 := OnOne2i_tl P i0 hd0 tl0 tl' b
    |})


DEBUG:Going to execute:
CoreTactics.intros_until_block