type message struct {
	seq int
	payload
}

class sequencer {
	var seq = 0

	// Receive a request from the FE.
	// Attach sequence number and multicast to RMs.
	on rUnicastReceive(payload) {
		rMulticastSend(message{seq, payload})
		seq++
	}
}

class receiver {
	var seq = 0
	var holdback Queue[message]

	// Receive from sequencer.
	on rMulticastReceive(m message) {
		append(holdback, m)
		trydeliver()
	}

	trydeliver() {
		if t.e. m in holdback s.t. m.seq == seq+1 {
			remove(m, holdback)
			deliver(m)
			seq++
			trydeliver()
		}
	}
}