The hidden track is a canon by Orlando di Lasso (1532-1594); since it itself is essentially a layered loop, I decided that I'd play it out once in Garageband, build out the canon in four voices, and then find loops in the loop library that worked with the harmonies in the canon. That turned out to be a jazzy bass, two trip-hop pads, and a bossa-nova piano. I built out the "cover" loops first, then brought in the canon, dropping out cover toward the end to get one full four-layer version of the canon as the voices finished up.
I ran across this canon in 1973, in Sir Donald Tovey's "Forms of Music"; this is the first time I've gotten to actually play it!
