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# Creating a game.
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Ideally one takes the [NNG template](https://github.com/hhu-adam/NNG4) to create a new game.
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## Game Structure
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A game consist of worlds which have multiple levels each. In the following we describe how to create a level file and how to combine these into a game.
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### Level
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#### Preample
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A level file is a lean file that imports at least `GameServer.Commands` and starts with the four lean commands
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```lean
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Game "NNG"
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World "Addition"
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Level 1
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Title "The rfl tactic"
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```
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Note that the levels inside a world must have consequtive numbering starting with `1`. The `Game`
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and `World` strings can be anything.
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#### Statement
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The core of a level is the `Statement`, which is the exercise that should be proven:
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```lean
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Statement MyNat.zero_add
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"For all natural numbers $n$, we have $0 + n = n$."
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(n : ℕ) : 0 + n = n := by
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Hint "You can start a proof by `induction n`."
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induction n with n hn
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· Hint "This is the base case."
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rw [add_zero]
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rfl
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· Hint "This is the induction hypothesis"
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rw [add_succ]
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Branch
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simp
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Hint "A branch is an alternative tactic sequence. Does not need to finish the proof."
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rw [hn]
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rfl
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```
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The first two arguments (name, attributes, and description) are optional. Thereafter you have a statement and a proof.
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- The proof should always be a tactic proof, i.e. the `:= by` is mandatory.
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- In the proof you can use `Hint "text"` to display text if the goal state in-game matches the one where `Hint` is placed. For more options about hints, see below.
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- In the proof you can add a `Branch` that runs an alternativ tactic sequence, which helps setting `Hints` in different places. The `Branch` does not affect the main proof and does not need to finish any goals.
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- If you specify a name (`MyNat.zero_add`), this lemma will be available in future levels. (Note that a future level must also import this level, so that Lean knows about the added statement). The name must be *fully qualified*.
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- If you add a description (i.e. non-Lean exercise statement), it will be shown above the exercise. It supports Markdown (including katex).
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#### Introduction/Conclusion
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Optionally, you can add a `Introduction "some text"` and `Conclusion "some text"` to your level. the introduction will always be visible on top, the conclusion is displayed once the level is solved.
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#### Lemmas/Tactics/Definitions
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Only enabled lemmas/tactics/definitions (called "items" here) are available in a level.
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To add a new item in a level, you can add
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```lean
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NewTactic rfl simp
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NewLemma MyNat.add_zero
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NewDefinition Nat
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```
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Once added, items will be available in all future levels/worlds, unless you disable them for a particular level with
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```lean
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DisabledTactic tauto
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DisabledLemma MyNat.add_zero
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```
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or specify explicitely which items should be available with
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```lean
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OnlyTactic rw rfl apply
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OnlyLemma MyNat.add_zero
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```
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Lastly, all items need documentation entries (which are imported in the level), see more about that below. There is also explains the `LemmaTab`
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### World
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Multiple levels are combined into a world and the worlds are then added to the game. It is recommended that all levels of a world are inside one folder (e.g. `NNG/Levels/Addition/`) and
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then there is one world file (`NNG/Levels/Addition.lean`) which contains the following
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```lean
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import NNG.Levels.Addition.Level_1
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import NNG.Levels.Addition.Level_2
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Game "NNG"
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World "Addition"
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Title "Addition World"
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Introduction "some text"
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```
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The `Title` is the world's display title. The `Introduction` is displayed before loading level 1.
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Note that all levels of a world should be imported by the world file and they **must not** be
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imported in another world's levels. (bug?)
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However, you can import an entire world in a different world's level: `import NNG.Levels.Addition`
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### Game
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The Game itself (i.e. the main file of you lake project, `NNG.lean`) should import all worlds and have the following layout, concluding with `MakeGame`:
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```lean
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import NNG.Levels.Addition
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import NNG.Levels.Multiplication
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import NNG.Levels.Power
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Game "NNG"
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Title "Natural Number Game"
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Introduction "some text"
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Path Addition → Multiplication → Power
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MakeGame
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```
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There can be several `Path` statements to define the relation between the worlds. The worlds
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must form a directed acyclic graph, i.e. have no loops. The order of worlds influences which tactics and lemmas will be unlocked in a given level.
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### Documentation
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Each tactic, lemma, or definition (all called items here) that is introduced in the game
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needs a documentation entry. These are statements of the following form:
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```lean
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LemmaDoc MyNat.add_squared as "add_squared" in "Pow"
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"(missing)"
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TacticDoc constructor
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"(missing)"
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DefinitionDoc One as "1"
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"(missing)"
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```
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Notes:
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* The lemma name must be **fully qualified**. The string display name can be arbitrary.
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* Tactics must have their proper name. use `TacticDoc «have» ""` if it does not work
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without french quotes.
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* Definition names can be arbitrary. E.g. I used `DefinitionDoc Symbol.Fun as "fun x ↦ x" "(missing)"` once.
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Moreover, the lemmas are in sorted in tabs (the `in "Pow`) part. In each level file, you
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can define which tab is open when the level is loaded by adding `LemmaTab "Pow"`.
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There will be features added to get automatic information from mathlib!
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