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
  H0 := {|
          pr1 := i0;
          pr2 :=
            {|
              pr1 := hd0;
              pr2 := {| pr1 := tl0; pr2 := {| pr1 := tl'0; pr2 := b |} |}
            |}
        |}
     : sigma
         (λ i : nat,
            sigma
              (λ _ : A,
                 sigma
                   (λ tl : list A,
                      sigma (λ tl' : list A, OnOne2i P (S i) tl tl'))))
  H : {|
        pr1 := i;
        pr2 :=
          {|
            pr1 := hd;
            pr2 := {| pr1 := tl; pr2 := {| pr1 := tl'; pr2 := a |} |}
          |}
      |} = H0
  ============================
   ({|
      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:
pattern sigma rhs
DEBUG LTAC Evaluated term: {|
                             pr1 := i0;
                             pr2 :=
                               {|
                                 pr1 := hd0;
                                 pr2 :=
                                   {|
                                     pr1 := tl0;
                                     pr2 := {| pr1 := tl'0; pr2 := b |}
                                   |}
                               |}
                           |}