code: public static BranchingReaderSource constructDocumentSource
          (ReaderConfig cfg, InputBootstrapper bs, String pubId, SystemId sysId,
           Reader r, boolean realClose) 
      {
          /* To resolve [WSTX-50] need to ensure that P_BASE_URL overrides
           * the defaults if/as necessary
           */
          URL url = cfg.getBaseURL();
          if (url != null) {
          	sysId = SystemId.construct(url);
          }
          BranchingReaderSource rs = new BranchingReaderSource(cfg, pubId, sysId, r, realClose);
          if (bs != null) {
              rs.setInputOffsets(bs.getInputTotal(), bs.getInputRow(),
                                 -bs.getInputColumn());
          }
          return rs;
      }
docs: Factory method used for creating the main-level document reader
  source.