Simple Interactive Calculator

Goal

Create a Console based Calculator in C#.

Rules

You can work in pairs if you like, but it's not required.
Everyone forks this repo (then clone your fork).
Everyone has until the beginning of class on Saturday to complete this task (this includes pushing to github)
There should be Unit Tests on all classes and methods you create.
Your solution should have at least 2 total projects. Unit Tests should be in it's own project.
This is an OOP focused class. Therefore, there will be classes. No implementations should be done within the Program class and Main method.
If a user submits an incomplete command or expression, the calculator should not attempt to evaluate it and print out a useful message.

How it should work

Starting your console application should create a prompt that looks like:

[x]>

where x is the number of executed commands/expressions during the user's current session. Call this your "counter".

On startup, your initial prompt should always look like:

[0]>

The user will enter simple mathematical expressions or commands. The user pressing [Enter] will have the calculator print out the correct answer on the following line (prefixed with an =). After the answer is printed, the user should return to the original prompt.

For example, below the user entered 2+1, then pressed [Enter]. The calculator printed out the answer 3 and reprinted the prompt, waiting for input from the user.

[0]> 2+1
   = 3
[1]>

Further use of this session will again increment the counter part of the prompt:

[1]> 1-10
   = -9
[2]>

Finally, typing exit or quit should exit the program while printing a message:

[2]> exit
Bye!!

Features

Math Operations

Your calculator should following operations

addition (integer)
subtraction (integer)
integer division (integer)
modulus (integer)
multiplication (integer)

Calculator Commands

In addition to math expressions, your calculator should accept the following commands:

quit and exit - exits the program
last - prints the last printed answer
lastq - prints the last entered command or expression

Constants

For the chosen few, you get the implement the concept of 'constants' in the caluculator. See below for how it should work:

[0]> x = 3
   = saved 'x' as '3'
[1]> x
   = 3
[2]> 1 + x
   = 4
[3]> x = 4
   = Error!

Constants Specifications

Any lowercase letter of the alphabet can be a constant.
Calculator should be case insensitive. A is considered the same as a.
Constants can only be defined once per session.
Defined constants can be used in math expressions
Undefined constants can not be used and if used should print a helful message.

C-Stein/simpleCalculatorJuniper