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
  H : {|
        pr1 := hd; pr2 := {| pr1 := tl; pr2 := {| pr1 := tl'; pr2 := a |} |}
      |} =
      {|
        pr1 := hd0;
        pr2 := {| pr1 := tl0; pr2 := {| pr1 := tl'0; pr2 := b |} |}
      |}
  ============================
   ({|
      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
    |})


DEBUG:Going to execute:
set (rhs := y) in *; pattern sigma rhs; clearbody rhs