code: private void ensureReturnFields(String[] requestedReturnFields) {
  
          if ((requestedReturnFields != null) && (requestedReturnFields.length > 0)) {
              List<String> result = new ArrayList<String>();
              for (String field : requestedReturnFields) {
                  String commasep = field.replaceAll(" ", ",");
                  List<String> list = CmsStringUtil.splitAsList(commasep, ',');
                  if (!list.contains("*")) {
                      for (String reqField : CmsStringUtil.splitAsList(MINIMUM_FIELDS, ",")) {
                          if (!list.contains(reqField)) {
                              list.add(reqField);
                          }
                      }
                  }
                  result.addAll(list);
              }
              setParam(CommonParams.FL, CmsStringUtil.arrayAsString(result.toArray(new String[0]), ","));
          }
      }
docs: Ensures that at least the 'path' and the 'type', 'id' and 'solr_id' are part of the fields returned field list.<p>
  
  @param requestedReturnFields the really requested return fields.
  
  @see CommonParams#FL