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
  H0 := {|
          pr1 := hd0;
          pr2 := {| pr1 := hd'0; pr2 := {| pr1 := tl0; pr2 := p0 |} |}
        |}
     : sigma (λ hd : A, sigma (λ hd' : A, sigma (λ _ : list A, P hd hd')))
  H : {|
        pr1 := hd; pr2 := {| pr1 := hd'; pr2 := {| pr1 := tl; pr2 := p |} |}
      |} = H0
  ============================
   ({|
      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:
pattern sigma rhs
DEBUG LTAC Evaluated term: {|
                             pr1 := hd0;
                             pr2 :=
                               {|
                                 pr1 := hd'0;
                                 pr2 := {| pr1 := tl0; pr2 := p0 |}
                               |}
                           |}