Sorry, no boolean traps allowed. Also, I'm not sure I want to special case strobe behaviour—shouldn't "off" just turn the whole thing "off"? 

I had some ideas about this...

- Return `off` to its previous state, where it doesn't kill the interval, but when called will simply "turn off" some state that can be checked in `strobe` and if "off", doesn't strobe, but allows the interval to continue. 
- Make `stop` the only method that can kill an interval.