Professor Appel was my boss when I was a system administrator for the UIUC Math department, working my way through grad school. He was one of the best bosses I’ve had over the past thirty years and had a real knack for dealing with the immature twenty something’s (like me).

He was incredibly humble about the 4 color proof, and well, just about everything. He continued to learn new things — I remember him asking me to help him debug some PostScript and I couldn’t believe it was happening.