If the value of ga is equal to 0, the below rule, will toggle the value between 0 and 1
ga=ga^1
This is an exclusive OR operator so what it does is it compares the current takes the current value and swaps it. It is a bitwise operator so in this case it works only on bit 0 of 32 bits.
See this post on bitwise operations.
Steve Caldwell
Bome Customer Care
Also available for paid consulting services: bome@sniz.biz