Kleene star operation
        Args:
            input_string (str): The string that the kleene star will be made
        Returns:
            str: The applied Kleene star operation on the input string

def star(self, input_string):
        """
        Kleene star operation
        Args:
            input_string (str): The string that the kleene star will be made
        Returns:
            str: The applied Kleene star operation on the input string
        """
        if input_string != self.epsilon and input_string != self.empty:
            return "(" + input_string + ")*"
        else:
            return ""