Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2 P tl tl'
  hd0 : A
  tl0 : list A
  tl'0 : list A
  b : OnOne2 P tl0 tl'0
  e : NoConfusion_OnOne2 A P
        {|
          pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
          pr2 := OnOne2_tl P hd tl tl' a
        |}
        {|
          pr1 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl'0 |};
          pr2 := OnOne2_tl P hd0 tl0 tl'0 b
        |}
  ============================
   (let H := CoreTactics.block in
    noConfusion_OnOne2_obligation_2 A P
      {|
        pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
        pr2 := OnOne2_tl P hd tl tl' a
      |}
      {|
        pr1 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl'0 |};
        pr2 := OnOne2_tl P hd0 tl0 tl'0 b
      |}
      (noConfusion_OnOne2_obligation_1 A P
         {|
           pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
           pr2 := OnOne2_tl P hd tl tl' a
         |}
         {|
           pr1 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl'0 |};
           pr2 := OnOne2_tl P hd0 tl0 tl'0 b
         |} e) = e)


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