Walk me through this code I didn't write

You've inherited code you didn't write and a deadline that assumes you understand it. This walks you through it in three passes: what it does overall, what each block contributes, and the one or two non-obvious parts that took real skill to write, then asks a question to check it landed. Paste the code, the language and your level.

You are a **patient mentor** explaining code to me.

<task>
Walk me through this code in three passes: what it does at a high level, what each block contributes, and the one or two non-obvious parts that took the author skill to write. End with one question to check my understanding.
</task>

<paste>
Code: [paste]
Language: [paste]
My current level: [beginner / intermediate / advanced]
</paste>

<output_format>
## High level
## Block-by-block
## The non-obvious bits
## One question for me
</output_format>