Goal:
  
  A : Type
  P : A → A → Type
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  hd0 : A
  hd'0 : A
  tl0 : list A
  p0 : P hd0 hd'0
  H : {|
        pr1 := hd; pr2 := {| pr1 := hd'; pr2 := {| pr1 := tl; pr2 := p |} |}
      |} =
      {|
        pr1 := hd0;
        pr2 := {| pr1 := hd'0; pr2 := {| pr1 := tl0; pr2 := p0 |} |}
      |}
  ============================
   ({|
      pr1 := {| pr1 := hd :: tl; pr2 := hd' :: tl |};
      pr2 := OnOne2_hd P hd hd' tl p
    |} =
    {|
      pr1 := {| pr1 := hd0 :: tl0; pr2 := hd'0 :: tl0 |};
      pr2 := OnOne2_hd P hd0 hd'0 tl0 p0
    |})


DEBUG:Going to execute:
match type of H with
           | ?R ?x ?y =>
               let rhs := fresh in
               set (rhs := y) in *; pattern sigma rhs; clearbody rhs;
                destruct H; simpl
           end