ok i understand now -- very low tech and simple concept, but it makes sense.
it also makes sense in terms of the difference in cognitive difficulty of recognition (easy) vs. recall (hard) -- that is, its relatively easy to *recognize* your pin when you see it, even if its mixed in with a lot of noise, vs. trying to just recall it from memory without any cues.
maybe if there you like to I can write a simple js code which would do all the filling
i dont think thats necessary -- the pdf should be sufficient for everyone.