Kenny Foner and I have a paper accepted to ICFP on subtracting bijections. Here’s the basic problem: suppose you have a bijection between two sum types, , and another bijection . Of course and must have the same size, but can you construct an actual bijection between and ?

This problem and its solution has been well-studied in the context of combinatorics; we were wondering what additional insight could be gained from a higher-level functional approach. You can get a preprint here.