Derivative

For finding the range, I am opting for method of differentiation to check intervals. hence, I have made derivative.py for differentiation.
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This is my second derivative calculator. The first one that I built was made completely using string slicing and weird string manipulation and pretty biased logic where everything was stored in the form of a dictionary. To be honest, I was pretty proud of that calculator because I was able to make it just using the knowledge I had, and recursion is not innate (I think) and I was still able to do it so I am proud of it. But, it was no doubt pretty inefficient with a lot of edge cases. Therefore, this time i am making a better calculator using my new AST representation and A LOT more recursion. This calculator will process more functions and it will be able to simplify the derivative too, for example: derivative of 2x^2 will not be displayed as 2*2x, rather it will be correctly displayed as 4x.