public static List<String> fields(final ExpressionOperator expressionOperator) {
        return terms(expressionOperator,
                null).stream().map(ExpressionTerm::getField).collect(Collectors.toList());
    }
@return True if there are at least one enabled term under the expressionOperator
stroom-query/stroom-query-api/src/main/java/stroom/query/api/ExpressionUtil.java