The whole point is that the gate-designer decides if their gate works in various directions (there are usually much more than 2 or 3). And this simply means that if they get a stream of events coming in in the wrong order, they can choose how to "complain": They can stay silent and ignore the broken language sent to them. Or they can throw a panic and halt the entire program. You decide. The NAND gate makes little sense to go in anything but one direction. But a PLUS gate or the REASON gate make sense in multiple directions. You can read more about this here: http://www.maymounkov.org/memex/abstract