Create a group of operations that may be launched in parallel.

template<
    typename... Ops>
parallel_group< Ops... > make_parallel_group(
    Ops... ops);

For example:

asio::experimental::make_parallel_group(
  [&](auto token)
  {
    return in.async_read_some(asio::buffer(data), token);
  },
  [&](auto token)
  {
    return timer.async_wait(token);
  }
).async_wait(
  asio::experimental::wait_for_all(),
  [](
      std::array<std::size_t, 2> completion_order,
      std::error_code ec1, std::size_t n1,
      std::error_code ec2
  )
  {
    switch (completion_order[0])
    {
    case 0:
      {
        std::cout << "descriptor finished: " << ec1 << ", " << n1 << "\n";
      }
      break;
    case 1:
      {
        std::cout << "timer finished: " << ec2 << "\n";
      }
      break;
    }
  }
);