@11011110 @j2kun TIL (from chasing down this post) about the Lambek-Moser Theorem, relating partitions of the natural numbers into two complementary sets, and nondecreasing unbounded functions on the natural numbers.
https://en.wikipedia.org/wiki/Lambek%E2%80%93Moser_theorem
How did I not know about this? The concept feels closely related to the proof that a set of natural numbers is computable iff it can be enumerated in non-decreasing order by a total Turing machine. Something I'm very familiar with; yet had never heard of Lambek-Moser.