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Lucas WintherandClaude Opus 5.5 820b378929 feat: undo in Module 9 and stackAt from Module 8; Lesson 8 quote matches the reference

Baba.undo() restores the level before the last step or move (board snapshot:
objects in age order with ids, the id counter, and won; rules recompiled).
place clears the history. SpellGrid's undo throws/panics 'not supported'.
stackAt / stack_at lists a tile's kinds oldest first, on Baba and SpellGrid.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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src

feat: undo in Module 9 and stackAt from Module 8; Lesson 8 quote matches the reference

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.course.json

feat: module 08 push chains

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.gitignore

feat: module 08 push chains

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package-lock.json

feat: module 08 push chains

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package.json

feat: module 08 push chains

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README.md

feat: undo in Module 9 and stackAt from Module 8; Lesson 8 quote matches the reference

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tsconfig.json

feat: module 08 push chains

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vitest.config.ts

feat: module 08 push chains

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README.md

Module 8 reference engine (TypeScript)

The course's own engine as of the end of Module 8. Part I's card engine is unchanged, and all of its tests still pass. The Part II engine in src/engine/puzzle/ now moves objects: each YOU object's move is planned in full, push chain and all, and only then committed. Module 7's reference was the starting point, so a diff between the two shows what this module adds.

cards/ (at the package root; not in this directory): Part I's card data, a copy of course/cards/ staged by tools/verify-references.mjs. Part II doesn't read it.
src/engine/puzzle/: the Part II engine. It names no noun: the engine's own words are the operators IS, AND, NOT, the meta-noun TEXT, the property PUSH of the base rule TEXT IS PUSH, and from Module 8 the property YOU, which picks the objects that move.
- grid.ts: the level format, object kinds and the board. Module 8 adds Board.find, Board.remove, Board.shift, Board.at and Kind.format (the kind as placed, for kindsAt).
- movement.ts (new): validate, then commit. Planner (tryMove, stayingAt, destroy, has), Step, Plan, Response and Dir. The planner reads the board and plans one step at a time; for each step it runs content's movement responses in order, and rolls back everything a step added if a response blocks it. The edge of the level blocks every step, and an object that has already stepped in the same move can't step again.
- level.ts: a level in play. Module 8 adds Level.moveYou: the YOU objects move one at a time, furthest ahead first, each planned and then committed, and an object steps at most once per move (a YOU object that an earlier move carried along doesn't start its own); the rules are reparsed once at the end. hasProperty is false for a destroyed object.
- word.ts, parse.ts, resolve.ts: unchanged.
- index.ts: the module's exports.
The rest of src/engine/: Part I, unchanged.
src/content/movement.ts (new) and src/content/index.ts: the movement responses, in the order the engine runs them: OPEN/SHUT, PUSH, STOP. What a property does when something moves is content, not engine.
src/content/words.json: Module 8 adds OPEN and SHUT.
src/harness.ts: the adapter the course tests call. Module 8 adds move, posOf, kindsAt and stackAt on Baba.

Run it with the module's tests: node tools/verify-references.mjs 08 from the course repository.