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'
  ============================
   (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:
try (intro; CoreTactics.intros_until_block)