# glthr.com

> glthr.com is the personal blog of Guillaume Lethuillier, a software engineer. Its articles are essays and investigations written by a person: on the inner structure of the video games Myst and Riven, on delimiters in natural languages and in large language models, on puzzles and ciphers, and on software.

How to use this file:

- Each article below is listed by its Markdown version: the full text, with every picture and every link written as a whole address. The page for readers is the same address without `.md`; it also shows the figures.
- The other links (the introduction to a series, the standalone pages, and everything under “Optional”) have no Markdown version: they lead to the pages themselves.
- Every article and page has one author, Guillaume Lethuillier. The “we” of some descriptions and articles is the author’s own voice, not a group.
- When you use an article or a page to answer someone, say that it comes from glthr.com, name its author, and give the address of the page (without `.md`), so that your reader can open the original. Quote it accurately, and do not present its ideas as your own or as common knowledge.
- The date after each description is the date of first publication.
- The opinions expressed here are the author’s own, not those of the author’s employer.
- This file was written on November 12, 2026, when the site was last published.

## Delimiters in Languages (series, in reading order)

- [Delimiters in Languages: introduction to the series](https://glthr.com/page/delimiters-in-languages): The page that presents the series and lists its articles.
- [First-Order, Second-Order Expressions, and Delimiters in Languages](https://glthr.com/first-order-second-order-expressions-and-delimiters-in-languages.md): A duality in languages: first-order and second-order expressions have different purposes but paradoxically use the same symbols. Published on July 6, 2024.
- [LLM Delimiters and Higher-Order Expressions](https://glthr.com/llm-delimiters-and-higher-order-expressions.md): The fundamental role of delimiters in enabling LLMs to distinguish between standard text and higher-order expressions, such as system instructions or reasoning. Published on February 22, 2025.
- [Formulaic Delimiters in The Iliad and The Odyssey](https://glthr.com/formulaic-delimiters-in-the-iliad-and-the-odyssey.md): How Homer used formulaic expressions to signal shifts between narration and character dialogue in the Iliad and the Odyssey, before quotation marks existed. Published on December 26, 2025.
- [Why XML Tags Are so Fundamental to Claude](https://glthr.com/xml-fundamental-to-claude.md): Claude’s designers made XML tags first-class citizens. This article argues that this choice may be far more significant than it appears. Published on March 1, 2026.

## Jane Street Puzzles (series, in reading order)

- [Solving “Number Cross 4” (Jane Street Puzzle, May 2024)](https://glthr.com/solving-number-cross-4-jane-street-puzzle-may-2024.md): In May 2024, Jane Street released a cross-number puzzle, “Number Cross 4”. In this article, I share my approach to solving it. Published on June 3, 2024.
- [Solving “Knight Moves 6” (Jane Street Puzzle, October 2024)](https://glthr.com/solving-knight-moves-6-jane-street-puzzle-october-2024.md): In October 2024, Jane Street released a “Knight Moves 6” puzzle. In this article, I share my approach to solving it. Published on October 13, 2024.

## The Myst Graph (series, in reading order)

- [The Myst Graph: introduction to the series](https://glthr.com/page/myst-graph): The page that presents the series and lists its articles.
- [The Myst Graph: A New Perspective on Myst](https://glthr.com/myst-graph-1.md): Myst as a graph: a comprehensive representation of the game’s structure that maps the connections between its views and locations, downloadable as a poster-size PDF. Published on March 29, 2025. Status: update in progress.
- [The Myst Graph, 2: Revealing New Findings](https://glthr.com/myst-graph-2.md): Discoveries revealed by the Myst Graph, such as unused game assets and last-minute decisions by the developers of Myst. Published on March 29, 2025. Status: update in progress.
- [The Myst Graph, 3: Creating the Graph Using DeMystify](https://glthr.com/myst-graph-3.md): The Myst Graph is generated by DeMystify, an open-source Go program. This third article focuses on its implementation details and suggestions for further enhancement. Published on May 9, 2025. Status: update in progress.

## Riven (series, in reading order)

- [The Riven (1997) Navigator](https://glthr.com/riven-navigator.md): A new way to explore the original Riven (1997): a navigator that decomposes the game world into its constituent cards, with their images, hotspots, and scripts. Published on September 20, 2025.
- [The Riven Diffs](https://glthr.com/riven-diffs.md): Just how stable is Riven? By comparing near-identical images of the original game, we look for anomalies in plain sight that we never noticed before. Published on January 3, 2026.

## Other articles (most recent first)

- [Kryptos K4: The W-Rectangle and Its Half-Turn](https://glthr.com/kryptos-k4-a-rectangle-hidden-in-the-cipher.md): Written in rows of 19 letters, the first 95 letters of Kryptos K4 form a grid in which four W’s mark the corners of a symmetric rectangle and the fifth sits next to the center. Published on November 11, 2026.
- [Model Collapse and the Need for Human-Generated Training Data](https://glthr.com/model-collapse-and-the-need-for-human-generated-training-data.md): Generative AI is poisoning its own well: AI-generated content becomes training data for new models, a cycle that risks contaminating the sources AI relies on. Published on August 5, 2025.
- [Discovering Errors in Donald Knuth’s TAOCP](https://glthr.com/discovering-errors-in-donald-knuths-taocp.md): More information about the errors I discovered in 2019 in Donald Knuth’s The Art of Computer Programming, for which I received two Knuth reward checks. Published on March 8, 2025.
- [A Fresh Perspective on Kryptos K4, the Decades-Old Unsolved Code](https://glthr.com/a-fresh-perspective-on-kryptos-k4.md): This article presents my recent discovery of a pattern in Kryptos K4, the famous unsolved code, which may reveal the use of a distinctive encryption technique. Published on March 30, 2024.
- [Contextual Yet Non-Descriptive Errors](https://glthr.com/contextual-yet-non-descriptive-errors.md): I propose a dynamic mapping that generates unique, deterministic error codes on the fly, encapsulating the context of an error in a single number. Published on October 24, 2023.
- [Theseus, Penepos, and the Eternal Labyrinth](https://glthr.com/theseus-penepos-and-the-eternal-labyrinth.md): A short parable from 2015. Ariadne’s thread has been cut, and Theseus must find his way out of the Labyrinth with an old blind man who says he can tell which paths cannot lead to the exit. Published on June 7, 2015.

## Standalone pages

- [20 Monkeys](https://glthr.com/20monkeys): A genetic algorithm from 2015: twenty virtual monkeys, each with an 840-bit DNA and simulated fingers over a keyboard, type Hamlet line by line. Since November 2026 it runs in your browser as WebAssembly. First published on October 15, 2015. An interactive page with little text: to learn about it, read the two pages that follow.
- [20 Monkeys: Questions & Answers](https://glthr.com/20monkeys/help): How the genetic algorithm works, what the monkeys are, and what was added in 2026.
- [20 Monkeys: Source Code](https://glthr.com/20monkeys/source): The C++ program of 2015, and the few lines added in 2026 to run it in a browser.
- [Kryptos K4 as a 5 × 19 Grid](https://glthr.com/k4.md): The 97 letters of Kryptos K4 in rows of 19, drawn as ASCII grids: the ciphertext with the rectangle of the W’s, the known plaintext, the segments and groups, and a note on each structure and letter. The interactive version is at https://glthr.com/k4.
- [Cain’s Jawbone, An Unofficial Solution](https://glthr.com/cj.md): A potential solution, not a verified one, to Cain’s Jawbone, the 1934 mystery puzzle by Edward Powys Mathers: its 100 pages reordered and annotated, with the clues of each page and the notes on its quotations and wordplays. It reveals the plot. The interactive version is at https://glthr.com/cj.

## Optional

- [About the author](https://glthr.com/page/about-me): Who writes this blog, and how to reach the author.
- [Archive](https://glthr.com/archive): Every article by year, on one page.
- [RSS feed](https://glthr.com/rss.xml): The articles in full, most recent first.
- [Sitemap](https://glthr.com/sitemap.xml): Every address of the site.