private Reader openRankingExpressionReader(String expName, String expression) {
        if (!expression.startsWith("file:")) return new StringReader(expression);

        String fileName = extractFileName(expression);
        Path.fromString(fileName); // No ".."
        if (fileName.contains("/")) // See ticket 4102122
            throw new IllegalArgumentException("In " + name() + ", " + expName + ", ranking references file '" +
                                               fileName + "' in a different directory, which is not supported.");

        return schema.getRankingExpression(fileName);
    }
Returns all filter fields in this profile and any profile it inherits.
config-model/src/main/java/com/yahoo/schema/RankProfile.java