The news of Marek's passing is immensely saddening. He was among the kindest, most thoughtful and optimistic people I have ever known. He was my first teacher of category theory and his influence over my outlook on mathematics cannot be overstated. Marek was also the one who urged me to take up a job at the University of Warsaw. I count myself lucky for having the benefit of his companionship over the last two years. He will be greatly missed.

Karol Szumiło