docs: private String getPortableFilename(BeanId beanId)
      {
          // Historically, this method returned the equivalent of
          // appendPortableFilenameString(beanId.toString()), where BeanId.toString
          // returned "BeanId(" + j2eeName + ", " + pkey + ")", which translates to
          // "BeanId_" + j2eeName + "__" + pkey + "_".                     RTC102568
  
          StringBuilder result = new StringBuilder();
          result.append(FILENAME_PREFIX);
          appendPortableFilenameString(result, beanId.getJ2EEName().toString());
          result.append("__");
          appendPortableFilenameString(result, beanId.getPrimaryKey().toString());
          result.append('_');
          return result.toString();
      }