code: private Path relPathToRoot(Path fullPath, Path root) {
      // just take some effort to do it 
      // rather than just using substring 
      // so that we do not break sometime later
      Path justRoot = new Path(Path.SEPARATOR);
      if (fullPath.depth() == root.depth()) {
        return justRoot;
      }
      else if (fullPath.depth() > root.depth()) {
        Path retPath = new Path(fullPath.getName());
        Path parent = fullPath.getParent();
        for (int i=0; i < (fullPath.depth() - root.depth() -1); i++) {
          retPath = new Path(parent.getName(), retPath);
          parent = parent.getParent();
        }
        return new Path(justRoot, retPath);
      }
      return null;
    }
docs: truncate the prefix root from the full path
  @param fullPath the full path
  @param root the prefix root to be truncated
  @return the relative path