server.start([&] (Socket socket) {
  std::async(std::launch::async, [&] (Socket sock) {
    socketstream stream(std::move(sock));
    {
      std::lock_guard<decltype(mutex)> lock(mutex);
      streams.insert(&stream);
    }
 
    std::string line;
    while (std::getline(stream, line)) {
      std::cout << line << '\n';
      std::lock_guard<decltype(*mutex)> lock(*mutex);
      for (auto* sockstream : streams) {
        if (sockstream == &stream) continue;
        *sockstream << line;
      }
    }
 
    streams.erase(&stream);
  }, std::move(socket));
});

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