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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 _forward(self):
 """Advance to the next token. Internal methods, updates: - self.current_token - self.current_pos Raises: MissingTokensError: when trying to advance beyond the end of the token flow. """

        try:
            self.current_token = next(self.tokens)
        except StopIteration:
            raise MissingTokensError("Unexpected end of token stream at %d." %
                self.current_pos)
        self.current_pos += 1