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
  H0 := {|
          pr1 := hd0;
          pr2 := {| pr1 := tl0; pr2 := {| pr1 := tl'0; pr2 := b |} |}
        |}
     : sigma
         (λ _ : A,
            sigma (λ tl : list A, sigma (λ tl' : list A, OnOne2 P tl tl')))
  H : {|
        pr1 := hd; pr2 := {| pr1 := tl; pr2 := {| pr1 := tl'; pr2 := a |} |}
      |} = H0
  ============================
   ({|
      pr1 := {| pr1 := hd :: tl; pr2 := hd :: tl' |};
      pr2 := OnOne2_tl P hd tl tl' a
    |} =
    {|
      pr1 :=
        {|
          pr1 := pr1 H0 :: pr1 (pr2 H0); pr2 := pr1 H0 :: pr1 (pr2 (pr2 H0))
        |};
      pr2 :=
        OnOne2_tl P (pr1 H0) (pr1 (pr2 H0)) (pr1 (pr2 (pr2 H0)))
          (pr2 (pr2 (pr2 H0)))
    |})


DEBUG:Going to execute:
<coq-core.plugins.ltac::clearbody@0> $1