docs: public static void skip(DataInputStream in) throws IOException {
          byte[] preamble = new byte[PREAMBLE.length];
          in.readFully(preamble);
          if (!Arrays.equals(preamble,PREAMBLE))
              return;    // not a valid preamble
  
          DataInputStream decoded = new DataInputStream(new UnbufferedBase64InputStream(in));
          int macSz = - decoded.readInt();
          if (macSz > 0) { // new format
              IOUtils.skip(decoded, macSz);
              int sz = decoded.readInt();
              IOUtils.skip(decoded, sz);
          } else { // old format
              int sz = -macSz;
              IOUtils.skip(decoded, sz);
          }
  
          byte[] postamble = new byte[POSTAMBLE.length];
          in.readFully(postamble);
      }