@THobson: Thanks for that tip - I've also fallen foul of this puzzle. You start with !alive, then blow the tracks of some BMP which stops moving and looks black but the trigger doesn't fire. So you change the trigger to !canmove, and the trigger tells you everything's ok while the BMP machine guns your squad.
Another wrinkle that caught me out: you can't use !alive with units who may not always be present. So if you use the random probability of having a unit, or if you have units not present in cadetmode (as I have), then you can't test for them being !alive - the result is not a boolean (it's some sort of null?).