You have been given a set of inference rules for propositional calculus. Using them, write a program containing a function that checks whether a given sequent is valid. Please call that function isValid. So isValid(s) should return True if is is a valid sequent and False if s is not valid.

You are required to have a type called Sequent for a sequent and a type called Formula for a propositional formula.

This program will not perform input and output. Test it using the top level, which allows you to evaluate expressions.