Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  b0 : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b0 hd hd'
  e : #|bs| = #|tl|
  b : B
  bs0 : list B
  hd0 : A
  tl0 : list A
  tl' : list A
  b1 : OnOne2All P bs0 tl0 tl'
  H : NoConfusion_OnOne2All A B P
        {|
          pr1 :=
            {|
              pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |}
            |};
          pr2 := OnOne2All_hd P b0 bs hd hd' tl p e
        |}
        {|
          pr1 :=
            {|
              pr1 := b :: bs0;
              pr2 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl' |}
            |};
          pr2 := OnOne2All_tl P b bs0 hd0 tl0 tl' b1
        |}
  ============================
   (let H := CoreTactics.block in
    {|
      pr1 :=
        {| pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |} |};
      pr2 := OnOne2All_hd P b0 bs hd hd' tl p e
    |} =
    {|
      pr1 :=
        {|
          pr1 := b :: bs0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl' |}
        |};
      pr2 := OnOne2All_tl P b bs0 hd0 tl0 tl' b1
    |})


DEBUG:Going to execute:
CoreTactics.intros_until_block