def process (self, input_symbol):

        '''This is the main method that you call to process input. This may
        cause the FSM to change state and call an action. This method calls
        get_transition() to find the action and next_state associated with the
        input_symbol and current_state. If the action is None then the action
        is not called and only the current state is changed. This method
        processes one complete input symbol. You can process a list of symbols
        (or a string) by calling process_list(). '''

        self.input_symbol = input_symbol
        (self.action, self.next_state) = self.get_transition (self.input_symbol, self.current_state)
        if self.action is not None:
            self.action (self)
        self.current_state = self.next_state
        self.next_state = None