code: private ArchivePath calculatePath(File root, File child) {
          String rootPath = unifyPath(root.getPath());
          String childPath = unifyPath(child.getPath());
          String archiveChildPath = childPath.replaceFirst(Pattern.quote(rootPath), "");
          return new BasicPath(archiveChildPath);
      }
docs: Calculate the relative child path.
  
  @param root
  The Archive root folder
  @param child
  The Child file
  @return a Path fort he child relative to root