Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  b0 : B
  bs : list B
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2All P bs tl tl'
  b : B
  bs0 : list B
  hd0 : A
  tl0 : list A
  tl'0 : list A
  b1 : OnOne2All P bs0 tl0 tl'0
  H : NoConfusion_OnOne2All A B P
        {|
          pr1 :=
            {|
              pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |}
            |};
          pr2 := OnOne2All_tl P b0 bs hd tl tl' a
        |}
        {|
          pr1 :=
            {|
              pr1 := b :: bs0;
              pr2 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl'0 |}
            |};
          pr2 := OnOne2All_tl P b bs0 hd0 tl0 tl'0 b1
        |}
  ============================
   (let H := CoreTactics.block in
    {|
      pr1 :=
        {| pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
      pr2 := OnOne2All_tl P b0 bs hd tl tl' a
    |} =
    {|
      pr1 :=
        {|
          pr1 := b :: bs0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl'0 |}
        |};
      pr2 := OnOne2All_tl P b bs0 hd0 tl0 tl'0 b1
    |})


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end