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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def wait_for_string(self, expected_string, timeout=60):
 """Wait for string FSM."""

        #                    0                         1                        2                        3
        events = [self.syntax_error_re, self.connection_closed_re, expected_string, self.press_return_re,
                  #        4           5                 6                7
                  self.more_re, pexpect.TIMEOUT, pexpect.EOF, self.buffer_overflow_re]

        # add detected prompts chain
        events += self.device.get_previous_prompts()  # without target prompt

        self.log("Expecting: {}".format(pattern_to_str(expected_string)))

        transitions = [
            (self.syntax_error_re, [0], -1, CommandSyntaxError("Command unknown", self.device.hostname), 0),
            (self.connection_closed_re, [0], 1, a_connection_closed, 10),
            (pexpect.TIMEOUT, [0], -1, CommandTimeoutError("Timeout waiting for prompt", self.device.hostname), 0),
            (pexpect.EOF, [0, 1], -1, ConnectionError("Unexpected device disconnect", self.device.hostname), 0),
            (self.more_re, [0], 0, partial(a_send, " "), 10),
            (expected_string, [0, 1], -1, a_expected_prompt, 0),
            (self.press_return_re, [0], -1, a_stays_connected, 0),
            # TODO: Customize in XR driver
            (self.buffer_overflow_re, [0], -1, CommandSyntaxError("Command too long", self.device.hostname), 0)
        ]

        for prompt in self.device.get_previous_prompts():
            transitions.append((prompt, [0, 1], 0, a_unexpected_prompt, 0))

        fsm = FSM("WAIT-4-STRING", self.device, events, transitions, timeout=timeout)
        return fsm.run()