Use with Locker, possible in a dedicated stack frame, like this:
// ... code ...
{ // open stack frame
	Locker locker(iu->payload());
	iu->payload()["a"] = "string";
	iu->payload()["b"] = "simultaneouslySentString";
} // close stack frame, sending the update

Address mode must be selected explicitly using as_map/as_list, like so:
  for (auto value: iu->payload()["listItem"].as_list()) { .... }
  for (auto kv_pair: iu->payload()["mapItem"].as_map()) { .... }
Generic size() function (reports 0 for non-structured types)

PayloadTypeConversionError thrown if (trimmed) string parse is
incomplete; comparison operators == and != catch this error though.