6.1: Outline the different types of messaging and define a scenario where each would be useful.
6.2: Extend the extension framework such that when a calculation is executed a message "Come back later" is returned. Also add to the extension a message "More data later." Neither message needs to be text based; both could be numerical. The objective is to be able to call an extension, get a message, and then come back to that session. Note the use of the word "session," which is a hint on how to solve the problem.
6.3: Implement the asynchronous messages using the Publish and Subscribe framework. Add an operation of your choosing that would ideally illustrate an asynchronous calculation.
6.4: Modify the calculator and associated extensions to use the stack principle of the HP calculator called HP Reverse Polish Notation (RPN). Hint: use the Command best practice and remember to clean up the entire calculator component (e.g., history, etc.).