Lesson learned: I wasted hours yesterday chasing a bug due to my failing to use either macros or higher-order functions to enforce a simple invariant in calling a recursion scheme over two tries (that they be the same height). It's not worth the saving of a function call!

Yesterday: find and fix the bug.
Today: make a future variant of this bug inexpressible.

Yet another bug where structural types couldn't help me, since it's about how the values of two data structures are related at runtime.

I looked at the OCaml code I ported to Scheme... it had this bug, and many other bugs I found. Types only help you so much.

What idiot wrote this buggy code?

This idiot.

More like, what idiot trusted his types too much and failed to test his code enough?