// data = output of jsonDecode()
sclass JSONKeyTree {
  new Set<S> out;
  
  Set<S> get(O data) {
    process("", data);
    ret out;
  }
  
  void process(S prefix, O data) {
    if (data cast L) {
      S s = prefix + "[]";
      out.add(s);
      for (O o : data)
        process(s, o);
    } else if (data cast Map) {
      for (O key, O val : data) {
        S s = prefix + "." + key;
        out.add(s);
        process(s, o);
      }
    } else if (data == null)
      out.add(prefix + " is null");
    else
      out.add(prefix + " is a " + shortClassName(o));
  }
}