code: @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));
      }
docs: 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