public static String relativeIfUnder(String path, String root) {
    if (!isParent(root, path)) return null;

    path = path.substring(root.length());
    if (path.length() == 0) return "";

    if (path.charAt(0) != DELIM) return null;
    return path.substring(1);
  }
public static String relativeIfUnder(String path, String root)