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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 customize_ruleset(self, custom_ruleset_file=None):
 """ Updates the ruleset to include a set of custom rules. These rules will be _added_ to the existing ruleset or replace the existing rule with the same ID. Args: custom_ruleset_file (optional):
 The filepath to the custom rules. Defaults to `None`. If `custom_ruleset_file` isn't passed, the environment variable `BOKCHOY_A11Y_CUSTOM_RULES_FILE` will be checked. If a filepath isn't specified by either of these methods, the ruleset will not be updated. Raises: `IOError` if the specified file does not exist. Examples: To include the rules defined in `axe-core-custom-rules.js`:: page.a11y_audit.config.customize_ruleset( "axe-core-custom-rules.js" ) Alternatively, use the environment variable `BOKCHOY_A11Y_CUSTOM_RULES_FILE` to specify the path to the file containing the custom rules. Documentation for how to write rules: https://github.com/dequelabs/axe-core/blob/master/doc/developer-guide.md An example of a custom rules file can be found at https://github.com/edx/bok-choy/tree/master/tests/a11y_custom_rules.js """

        custom_file = custom_ruleset_file or os.environ.get(
            "BOKCHOY_A11Y_CUSTOM_RULES_FILE"
        )

        if not custom_file:
            return

        with open(custom_file, "r") as additional_rules:
            custom_rules = additional_rules.read()

        if "var customRules" not in custom_rules:
            raise A11yAuditConfigError(
                "Custom rules file must include \"var customRules\""
            )

        self.custom_rules = custom_rules