// data = output of jsonDecode()
sclass JSONKeyTree {
  new TreeSet<S> out;
  new MultiMap<S, O> examples;
  
  S render(O data) {
    process(data);
    ret lines(out, key -> {
      var xmpls = examples.get(key);
      ret key + (empty(xmpls) ? "" : ", e.g. " + jsonEncode(first(xmpls)));
    });
  }
  
  Set<S> get(O data) {
    process(data);
    ret out;
  }
  
  void process(S prefix default "", O data) {
    if (data cast L) {
      out.add(prefix + " is a list");
      S s = prefix + "[]";
      for (O o : data)
        process(s, o);
    } else if (data cast Map) {
      out.add(prefix + " is a map");
      for (O key, O val : castMapToMapO(data)) {
        S s = addDotIfNempty(prefix) + key;
        out.add(s);
        process(s, val);
      }
    } else if (data == null)
      out.add(prefix + " is null");
    else {
      out.add(prefix + " is a " + shortClassName(data));
      examples.put(prefix, data);
    }
  }
}