docs: public List<String> list(String folder) throws IOException {
        List<String> res = new ArrayList<String>();
        if (path != null) {
          File f = new File(Utilities.path(path, folder));
          if (f.exists() && f.isDirectory())
            for (String s : f.list())
              res.add(s);
        } else {
          for (String s : content.keySet()) {
            if (s.startsWith(folder+"/") && !s.substring(folder.length()+2).contains("/"))
              res.add(s.substring(folder.length()+1));
          }
        }
        return res;
      }