Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  hd0 : A
  tl0 : list A
  tl' : list A
  b : OnOne2 P tl0 tl'
  ============================
   (let H := CoreTactics.block in
    NoConfusion_OnOne2 A P
      {|
        pr1 := {| pr1 := hd :: tl; pr2 := hd' :: tl |};
        pr2 := OnOne2_hd P hd hd' tl p
      |}
      {|
        pr1 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl' |};
        pr2 := OnOne2_tl P hd0 tl0 tl' b
      |}
    → let H0 := CoreTactics.block in
      {|
        pr1 := {| pr1 := hd :: tl; pr2 := hd' :: tl |};
        pr2 := OnOne2_hd P hd hd' tl p
      |} =
      {|
        pr1 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl' |};
        pr2 := OnOne2_tl P hd0 tl0 tl' b
      |})


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 5: T
 has value 
(NoConfusion_OnOne2 A P
   {|
     pr1 := {| pr1 := hd :: tl; pr2 := hd' :: tl |};
     pr2 := OnOne2_hd P hd hd' tl p
   |}
   {|
     pr1 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl' |};
     pr2 := OnOne2_tl P hd0 tl0 tl' b
   |}
 → let H := CoreTactics.block in
   {|
     pr1 := {| pr1 := hd :: tl; pr2 := hd' :: tl |};
     pr2 := OnOne2_hd P hd hd' tl p
   |} =
   {|
     pr1 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl' |};
     pr2 := OnOne2_tl P hd0 tl0 tl' b
   |})