Of course it can be discovered if you look for it. What was impressive about the folks who got Thompson's compiler at PWB is that they found the horse even though they weren't looking for it. Then there was the first time Jim Reeds and I turned on integrity control in IX, our multilevel-security version of Research Unix. When it reported a security violation during startup we were sure it was a bug. But no, it had snagged Tom Duff's virus in the act of replication. It surprised Tom as much as it did us, because he thought he'd eradicated it. Doug