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'
  idx3 : nat
  idx4 : list A
  idx2 : list A
  b : OnOne2i P idx3 idx4 idx2
  ============================
   (∀ (H := CoreTactics.block) (e : NoConfusion_OnOne2i A P
                                      {|
                                        pr1 :=
                                          {|
                                            pr1 := i;
                                            pr2 :=
                                              {|
                                                pr1 := hd :: tl;
                                                pr2 := hd :: tl'
                                              |}
                                          |};
                                        pr2 := OnOne2i_tl P i hd tl tl' a
                                      |}
                                      {|
                                        pr1 :=
                                          {|
                                            pr1 := idx3;
                                            pr2 :=
                                              {| pr1 := idx4; pr2 := idx2 |}
                                          |};
                                        pr2 := b
                                      |}),
      let H0 := CoreTactics.block in
      noConfusion_OnOne2i_obligation_2 A P
        {|
          pr1 :=
            {| pr1 := i; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
          pr2 := OnOne2i_tl P i hd tl tl' a
        |}
        {|
          pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
          pr2 := b
        |}
        (noConfusion_OnOne2i_obligation_1 A P
           {|
             pr1 :=
               {| pr1 := i; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
             pr2 := OnOne2i_tl P i hd tl tl' a
           |}
           {|
             pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
             pr2 := b
           |} e) = e)


DEBUG:Going to execute:
fun tac p =>
             let ty := type of p in
             let eliminator := eval simpl in DepElim.elim in
             tac p eliminator