BLATANT RIP
This is a blatant rip of Andy Naughton's,
"The Flash Mind Reader"
(and he does a MUCH better job at this - with no CLICK on solution (pay me) to see how it works).
Check the dates each were written.
Okay, the solution. (Spoiler!)
No matter what symbol you choose, it is inserted every 9 frames. (Look at frame 0, 9, 18, etc..)
It's a neat trick but GSPZ doesn't deserve credit for it.
GSPZ, the least you could do is either credit ANDY or the famous mathematician that developed this !
David