protected function getCustomizedRuleKeysToKeepDefault(array $customRules)
    {
        $keys = [];

        foreach ($customRules as $key => $ruleParts) {

            // If the configured value is the name of a key (and so it is a non-associative
            // part of the rules), it indicates that the default should be kept/copied over,
            // for the key the value denotes.
            if (is_string($ruleParts) && is_numeric($key)) {
                $keys[] = $ruleParts;
            }
        }

        return $keys;
    }