This can be useful for doing trial receptions of messages, and
if they work, update the state when successful...  Setting the
state on the old object is best as it's hard to change all references
to the new object..

copyof = strb.copy()
try:
	copyof.recv_enc(msg)
	copyof.recv_mac(mac)
except AuthenticationFailed: # failed, ignore message
	return
else: # it worked
	strb.set_state_from(copyof)

<process msg>