Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  idx2 : list A
  idx1 : list A
  b : OnOne2 P idx2 idx1
  H : NoConfusion_OnOne2 A P
        {|
          pr1 := {| pr1 := hd :: tl; pr2 := hd' :: tl |};
          pr2 := OnOne2_hd P hd hd' tl p
        |} {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |}
  ============================
   (let H := CoreTactics.block in
    {|
      pr1 := {| pr1 := hd :: tl; pr2 := hd' :: tl |};
      pr2 := OnOne2_hd P hd hd' tl p
    |} = {| pr1 := {| pr1 := idx2; pr2 := idx1 |}; pr2 := b |})


DEBUG:Going to execute:
lazymatch goal with
           | H:_ |- _ => tac H
           end