thought you might all get a giggle from this.
I have a piston that sets home occupied mode to a simple true or false which in turn controls many things including the arming of my alarm.
I wrote the piston like this
if any presence sensors are present
----- turn home occupied to true
else
----- turn home occupied to off
Guess what happened when I left the house this morning and left my wife at home 
(lets just say it was a confusing time for the whole house, including all the pistons)