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 := hd0 :: tl0; pr2 := hd0 :: tl'0 |};
      pr2 := OnOne2_tl P hd0 tl0 tl'0 b
    |})


DEBUG:Going to execute:
pattern sigma rhs
DEBUG LTAC Evaluated term: {|
                             pr1 := hd0;
                             pr2 :=
                               {|
                                 pr1 := tl0;
                                 pr2 := {| pr1 := tl'0; pr2 := b |}
                               |}
                           |}