+Empty IOperation
+Evaluate method to interface
+Empty Adder, not-implemented
+Evaluate implementation for Adder
+Empty Calculator
+Ability to Add to Calculator
+Empty Subtractor, not Implemented
Implement Evaluate in Subtractor
+Ability to Subtract to Calculator
+Empty Multiplier operation
Implement Evaluate in Multiplier
+Empty Divider operation
Implement Evaluate in Divider
Add ability to Multiply to Calculator
Add ability to Divide to Calculator
+Undo to the Calculator
+Redo to the Calculator