// grabs only the specified keys from the map.
static S hiddenFields(Map<S, S> map, Collection<S> keys) {
  new StringBuilder buf;
  for (S key : keys) {
    S value = map.get(key);
    if (!empty(value))
      buf.append(htag("input", "", "type", "hidden", "name", key, "value", value) + "\n");
  }
  ret str(buf);
}