Display negative exponents always as fraction

Question

I want Mathematica to display negative exponents always as fractions. I already tried:

Needs["Notation`"]
(* Notation[1/a_^b_ <==> a_^-b_] *)
Notation[
  ParsedBoxWrapper[RowBox[{"1", "/", RowBox[{"a_", "^", "b_"}]}]]
  ⟺ 
  ParsedBoxWrapper[RowBox[{"a_", "^", RowBox[{"-", "b_"}]}]]]

While I do have

x^-z

1/x^z 

the following doesn't work:

x^-(2z)

x^-2z

Some hints ? Thank you very much !

Answer 1

I found a working solution for my case. It still uses the Notation Package and makes use of PatternWrappers:

Here is the Code:

Needs["Notation`"]
(*Notation[1/Simplify[a_^(-b_ c_)] <==> a_^TemplateBox[b_?Negative] c_]*)
(*Notation[(a_^d_)/Simplify[a_^(-b_ c_)] <==> a_^TemplateBox[b_?Negative] c_+d_]*)
ClearNotations[]
Notation[ParsedBoxWrapper[
FractionBox["1", 
SuperscriptBox["", 
TemplateBox[{
RowBox[{"Simplify", "[", 
SuperscriptBox["a_", 
RowBox[{
RowBox[{"-", "b_"}], " ", "c_"}]], "]"}]}, 
  "NotationPatternTag"]]]] \[DoubleLongLeftArrow] ParsedBoxWrapper[
SuperscriptBox["a_", 
RowBox[{
TemplateBox[{
RowBox[{"b_", "?", "Negative"}]}, "NotationPatternTag"], " ", "c_"}]]]]
Notation[ParsedBoxWrapper[
FractionBox[
SuperscriptBox["a_", "d_"], 
TemplateBox[{
RowBox[{"Simplify", "[", 
SuperscriptBox["a_", 
RowBox[{
RowBox[{"-", "b_"}], " ", "c_"}]], "]"}]}, 
  "NotationPatternTag"]]] \[DoubleLongLeftArrow] ParsedBoxWrapper[

SuperscriptBox["a_", 
RowBox[{
RowBox[{
TemplateBox[{
RowBox[{"b_", "?", "Negative"}]}, "NotationPatternTag"], " ", "c_"}], 
   "+", "d_"}]]]]