@Override
    public Q addFlag(Position position, String prefix, Expression<?> expr) {
        Expression<?> flag = Expressions.template(expr.getType(), prefix + "{0}", expr);
        return queryMixin.addFlag(new QueryFlag(position, flag));
    }
Add the given prefix and expression as a general query flag

@param position position of the flag
@param prefix prefix for the flag
@param expr expression of the flag
@return the current object
querydsl-sql/src/main/java/com/querydsl/sql/ProjectableSQLQuery.java