Odyssee

The message mode + causes change to subsequently send out a received number
only if it is greater than the previously received number; that is, only an
increasing progression of numbers can be sent out. The message mode - causes
change to subsequently send out a received number only if it is less than the
previously received number; only a decreasing progression of numbers can be
sent out. The word mode by itself causes change to send out any received
number, provided it is different from the previously received value.