docs: public void deserialize(DataTree dt, Map<Long, Long> sessions,
              InputArchive ia) throws IOException {
          FileHeader header = new FileHeader();
          header.deserialize(ia, "fileheader");
          if (header.getMagic() != SNAP_MAGIC) {
              throw new IOException("mismatching magic headers "
                      + header.getMagic() +
                      " !=  " + FileSnap.SNAP_MAGIC);
          }
          SerializeUtils.deserializeSnapshot(dt,ia,sessions);
      }