void
atm::getting_pin()
{
  incoming.wait()
      .handle<digit_pressed>([&](digit_pressed const& msg) {
        unsigned const pin_length = 4;
        pin += msg.digit;
        if (pin.length() == pin_length) {
          bank.send(verify_pin(account, pin, incoming));
          state = &atm::verifying_pin;
        }
      })
      .handle<clear_last_pressed>([&](clear_last_pressed const& msg) {
        if (!pin.empty()) {
          pin.resize(pin.length() - 1);
        }
      })
      .handle<cancel_pressed>(
          [&](cancel_pressed const& msg) { state = &atm::done_processing; });
}