For his paper on his language Cayenne. He was also cleverly wearing this t-shirt!
Yay for nerds!
(Btw, I help him come up with the weird names for his programming languages. Haskell (not one of Lennart's, but very used and influenced and reworked by him) was named after Haskell Curry. And somebody had written a language called Curry. So Lennart wanted something spicy. I suggested Cayenne. He has another language he's written that he said was a bit magical, but he didn't want to call it Wizard or Magician, so I said, "How about Djinn?" and Djinn it was! :) )