Who? --The real question is, "What'd he do?" and the answer is "Compose." See, I'd looked up Paul Erdos,* the famously eccentric mathematician and that'd led me to Glenn Gould, the famously talented (and eccentric) Canadian pianist, and he was a big fan of Sweelnick despite the composer's having left† nearly 400 years earlier.
The lengthy samples of Jan Pieterszoon Sweelinck's work on his Wikipedia page are pleasant and delightfully complex, pre-Bach but resolutely headed that way, the sort of music that sounds like heaven and takes a hell of a lot of work to play properly.
Organ music: it ain't just for the hockey stadium and the roller rink, y'know.
* It's really "Erdős," but in these days of ITAR and renewed tensions near that part of Europe, who's got an o with a double-acute accent handy?
† An Erdős-ism meaning "died."
3 months ago