What is cosh(List(Bool))? is my favorite blog post of 2025. Not only did it inspire my data type derivatives post, but I also wrote a final paper this semester about a related topic, combinatorial species, which provide a deep, rigorous theory to justify the handwavey proofs in those blog posts. I also included a simple new proof of Cayley’s formula using the species formulation of the Lagrange inversion theorem rather than the real analysis version.