workind calculator with exponent, sqrt, invert and square

src/main/java/rocks/zipcode/calcskin/CalcSkin.java

     private DoubleProperty currentValue = new SimpleDoubleProperty();
     private CalcEngine calcEngine = new CalcEngine();
 
-    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE, SQUARED, SQUAREROOT, INVERT, EXPONENT  }
+    private enum Op { NOOP, ADD, SUBTRACT, MULTIPLY, DIVIDE, EXPONENT  }
 
     private Op curOp   = Op.NOOP;
     private Op stackOp = Op.NOOP;
         screen.setStyle("-fx-background-color: aquamarine;");
         screen.setAlignment(Pos.CENTER_RIGHT);
         screen.setEditable(false);
-        screen.textProperty().bind(Bindings.format("%.0f", currentValue));
+        screen.textProperty().bind(Bindings.format("%.4f", currentValue));
         return screen;
     }
 
                 makeSquareButton(button);
             } else if ("sqrt".equals(s)){
                 makeSquareRootButton(button);
-            } else if ("i/x".equals(s)){
+            } else if ("1/x".equals(s)){
                 makeInvertButton(button);
             }
         }