code: def customize_ruleset(self, custom_ruleset_file=None):
          
          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
docs: 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