Goal:
  
  A : Type
  P : nat → A → A → Type
  i : nat
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2i P (S i) tl tl'
  i0 : nat
  hd0 : A
  tl0 : list A
  tl'0 : list A
  b : OnOne2i P (S i0) tl0 tl'0
  H : {|
        pr1 := i;
        pr2 :=
          {|
            pr1 := hd;
            pr2 := {| pr1 := tl; pr2 := {| pr1 := tl'; pr2 := a |} |}
          |}
      |} =
      {|
        pr1 := i0;
        pr2 :=
          {|
            pr1 := hd0;
            pr2 := {| pr1 := tl0; pr2 := {| pr1 := tl'0; pr2 := b |} |}
          |}
      |}
  ============================
   ({|
      pr1 := {| pr1 := i; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
      pr2 := OnOne2i_tl P i hd tl tl' a
    |} =
    {|
      pr1 :=
        {| pr1 := i0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl'0 |} |};
      pr2 := OnOne2i_tl P i0 hd0 tl0 tl'0 b
    |})


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