F91 in Lean

Back in March, with version 4.17.0, Lean introduced partial_fixpoint , a new way to define recursive functions. I had drafted a blog post for the official Lean FRO blog back then, but forgot about it, and with the Lean FRO blog discontinued, I’ll just publish it here, better late than never. With the partial_fixpoint mechanism we can model possibly partial functions (so those returning an Option )…