private void commaSeparated(StringBuilder yql, Set<String> fields) {
        int initLen = yql.length();
        for (String field : fields) {
            if (yql.length() > initLen) {
                yql.append(", ");
            }
            yql.append(field);
        }
    }
Serialize this query as YQL+. This method will never throw exceptions,
but instead return a human readable error message if a problem occurred while
serializing the query. Hits and offset information will be included if
different from default, while linguistics metadata are not added.

@return a valid YQL+ query string or a human readable error message
@see Query#yqlRepresentation(boolean)
container-search/src/main/java/com/yahoo/search/Query.java