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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 eval_string(self, s):
 """ Returns the tristate value of the expression 's', represented as 0, 1, and 2 for n, m, and y, respectively. Raises KconfigError if syntax errors are detected in 's'. Warns if undefined symbols are referenced. As an example, if FOO and BAR are tristate symbols at least one of which has the value y, then config.eval_string("y && (FOO || BAR)") returns 2 (y). To get the string value of non-bool/tristate symbols, use Symbol.str_value. eval_string() always returns a tristate value, and all non-bool/tristate symbols have the tristate value 0 (n). The expression parsing is consistent with how parsing works for the C implementation. m is rewritten to 'm && MODULES', so eval_string("m") will return 0 (n) unless modules are enabled. """

        # The parser is optimized to be fast when parsing Kconfig files (where
        # an expression can never appear at the beginning of a line). We have
        # to monkey-patch things a bit here to reuse it.

        self._filename = None

        # Don't include the "if " from below to avoid giving confusing error
        # messages
        self._line = s
        self._tokens = self._tokenize("if " + s)
        self._tokens_i = 1  # Skip the 'if' token

        return expr_value(self._expect_expr_and_eol())