Say thank you to your mentors. You never know when it's the last time you'll see them.

Hans was the professor of the first CS class I ever TA'd, an Introduction to C++ course at Northwestern for gifted teenagers during the summer of 2004. I had just turned 18 and was frequently mistaken for a student rather than a TA, which made me feel even more out of place with the conspicuously older TAs who would occasionally go to bars together in the evening.

So I ended up hanging out with Hans. He was a patient, flexible teacher more interested in the growth of his students than in adherence to a syllabi. We quickly came to trust each other's knowledge of the subject matter and turn our attention from structs and pointers to the far more pressing matter of how to help the human beings in our classroom every day: why was this obviously brilliant boy acting out? What to do with a student who refused to shed old, ineffective programming habits when they started impacting his ability to write more complex code? How to encourage the students who were enthusiastically racing ahead in the unexpected freedom of a project-based classroom?

His general teaching philosophy was to get students excited, give them support, and get out of their way -- but the support was there for those who needed it. There was one quiet student, the only girl in the class, who was clearly holding back; when the classroom turned open-ended, it was almost as if she was afraid to strike out on her own. Hans and I talked about it, and began working with her privately in the afternoons after classes were over, teaching her different programming langauges, slowly coaxing her out of her shell. The girl's parents arrived for parent-teacher conferences, focused on grades and test scores, external metrics of achievement. Push the child to go faster, collect more badges evidencing her intelligence -- never mind how she felt about it, or whether she was learning anything along the way. When he spoke with them, Hans could barely contain his thunder; you have an extraordinary daughter who will go so much farther if you give her room to be who she is. Please give her at least some intellectual freedom; she's not even in high school yet. Tend to her spirit as much as her mind. They left, seemingly unaffected; I saw the anguish in Hans's face afterwards. We talked about how the most difficult part of loving your students was letting them go, sometimes into a world that wouldn't nurture them as much as one would hope for good children, filled with such promise, but still so fragile forming and so easily breakable.

He gave me a huge stack of his textbooks, canonical tomes on algorithms and artificial intelligence that I read hungrily that summer, though I couldn't understand many of the ideas at that time. The same books saved me a great deal of money later in college when I found they were required texts for one of my CS classes. (They've since been passed on to Sebastian.) But the main thing Hans taught me was his teaching philosophy. His stance towards education was a radical one, and my teenage self was thrilled to hear an adult speaking out against the brokenness of educational institutions, sometimes infuriated, always impassioned. Why grades? Why college? Why these restrictions, why these ideals? He challenged my preconceptions of the messages I'd heard, asking me who stood to benefit from each line of education-related propaganda or advertising I had heard. But he never oversimplified it to a battle of good vs. evil; he also tried to explain the history and politics behind decisions, acknowledging that he couldn't see the full picture himself. When I started reading authors like Alfie Kohn and John Holt and Neil Postman the following school year, their writings made sense partially because I could ground them in my experiences of Hans's classroom, our long talks outside of class.

He was a jack-of-all-trades, and could talk literature, auto mechanics, bikes, and poetry as well as he could computer science. I saw that it was possible to live a joyful, rich life without singular devotion to a narrow specialty; plunging into whatever suited you at a given time was something that could be done, and moving on to other things when they interested you more did not mean that you'd never get deeply involved in anything. I won't say Hans was a perfect human being; he wasn't. (Nobody is.) I don't have an idealized vision of him -- he could be gruff, come off as too strong a radical to people in authority, and his convictions combined with his wanderlust kept him from going deeply enough into any academic discipline to make new research contributions to the field, no matter how strongly he mastered the existing material -- but that too was a teaching, a lesson for a young firebrand. I could easily flare out with that kind of fire all my life, but this would be the tradeoff. Learning temperance would be a tradeoff too. What balance of discipline did I want to find?

We kept talking after the summer, meeting with former students in a cafe near Northwestern's campus on a few occasions to geek out about our continued adventures in computing. But then college got busy and I fell out of touch. The last time I saw him was the summer of 2008, years after we'd last spoken; he visited the ILXO office during one of our OLPC workshops, and I blinked at how different things seemed then. I say this next bit with some guilt and nervousness that it may come across as arrogant -- I felt as if I had outgrown him, grown past him. The man who seemed like such a thundering giant to me at 18 looked different to my 22-year-old self; a brilliant man relegated to the ordinary by those who didn't know him, largely because he resisted settling down into the temperance and focus that institutions tend to require. But despite -- or really because -- of his flaws and his virtues combined, he shaped a crucial part of my young life.

I was writing up my CV for a fellowship application this morning, came across the entry about TAing his class, and thought "hey, I wonder what he's up to now; I should go get in touch now that I'm back within driving distance of where he lives." So I looked -- and found that he had succumbed to cancer at the end of 2010, the same time I was writing my application for graduate school. I had planned to write a letter to Hans saying hello and thank you, asking if we could catch up. I guess this is what I ended up writing instead.

I'm sorry we never got to get another cup of lentil soup together at the Central Street Cafe, Hans. Thank you.