private static ExpressionNode parse(Reader reader) throws ParseException {
        try {
            return new RankingExpressionParser(reader).rankingExpression();
        }
        catch (TokenMgrException e) {
            throw new ParseException(e.getMessage());
        }
    }
Parses the content of the reader object as an expression string.

@param reader A reader object that contains an expression string.
@return An expression node that corresponds to the given string.
@throws ParseException if the string could not be parsed.
searchlib/src/main/java/com/yahoo/searchlib/rankingexpression/RankingExpression.java