@att It's actually ok once you dive in -- the hard bit is testing; there are so many possible combinations, and covering all corner cases is likely where slip-ups will happen.
@Richard The nice thing with Base85 is that there are a gazillion websites that offer a form for testing your encode and decode results.
@Adám I think your description on youtube is for a different episode (Write a dfn that removes extraneous (leading, trailing, and multiple) spaces from a character vector.)
@xpqz Morten just mentioned "Learning APL" at the Finnish and German user group meetups, which got me thinking; what happened with xpqz.github.io/cultivations?