Appendix C, Listing C9

[HlodricIB]

Reading Appendix C, A message-passing framework and complete ATM example, I could not find where the mutex iom is created, which is used in Listing C9, for example here

            incoming.wait()
                .handle<issue_money>(
                    [&](issue_money const& msg)
                    {
                        {
                            std::lock_guard<std::mutex> lk(iom);
                            std::cout<<"Issuing "
                                     <<msg.amount<<std::endl;
                        }
                    }
                    )

Am I missing something?