@Override
public Object answer(InvocationOnMock invocation) throws Throwable {
Envelope env = (Envelope) invocation.getArguments()[0];
ChannelID channelId = env.getSource();
int seqNum = env.getSequenceNumber();
if (seqNum == 0) {
Assert.assertNull(
String.format("Received envelope from %s before, but current seq num is 0", channelId),