code: public static String repeatQM(int n) {
          N.checkArgNotNegative(n, "count");
  
          String result = QM_CACHE.get(n);
  
          if (result == null) {
              result = StringUtil.repeat("?", n, ", ");
          }
  
          return result;
      }
docs: Repeat question mark({@code ?}) {@code n} times with delimiter {@code ", "}.
  <br />
  It's designed for batch SQL builder.
  
  @param n
  @return