migrate all tutorials from Markdown to Org mode

Converted with pandoc 3.7 (markdown → org), all 17 files:
- README, references, 15 tutorial units
- Internal file links updated from .md to .org
- Source code blocks (#+begin_src lean) preserved
- Tables, math notation, links intact

For the Emacs workflow.

tutorial-02-semantics/08-syntax-representation.org

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+* Unit 8 --- Representing Syntax
+:PROPERTIES:
+:CUSTOM_ID: unit-8-representing-syntax
+:END:
+*Tutorial 2: PL Semantics in Lean* · [[../README.org][← Back to README]]
+
+** Goals
+:PROPERTIES:
+:CUSTOM_ID: goals
+:END:
+- Encode lambda calculus terms as an inductive type
+- Understand three binding representations:
+  1. *Named* (strings --- simple, but α-equiv isn't definitional)
+  2. *de Bruijn indices* (numbers --- α-equiv is free, shifting is painful)
+  3. *Locally nameless* (free vars named, bound vars indexed --- compromise)
+
+We'll use de Bruijn indices (the "heavy lifter") for the rest of this tutorial,
+with locally nameless for comparison.
+
+** Sources
+:PROPERTIES:
+:CUSTOM_ID: sources
+:END:
+- syndikos/lean4-stlc =Syntax.lean=: https://github.com/syndikos/lean4-stlc
+- Chris Henson 2025: https://chrishenson.net/posts/2025-05-10-formalized_lambda_calculus.html
+- chenson2018/LeanScratch: https://github.com/chenson2018/LeanScratch
+- Software Foundations Vol.2: https://softwarefoundations.cis.upenn.edu/
+
+** Exercises
+:PROPERTIES:
+:CUSTOM_ID: exercises
+:END:
+#+begin_src lean
+-- 8.1 — Named representation
+inductive NamedTerm where
+  | var (x : String)
+  | lam (x : String) (body : NamedTerm)
+  | app (f arg : NamedTerm)
+deriving Repr
+
+-- The Church encoding of identity: λx. x
+def idNamed : NamedTerm := NamedTerm.lam "x" (NamedTerm.var "x")
+
+-- Encode λx. λy. x  (K combinator)
+def kNamed : NamedTerm :=
+  sorry
+
+-- Encode λf. λx. f (f x)  (Church numeral 2)
+def twoNamed : NamedTerm :=
+  sorry
+
+-- 8.2 — de Bruijn representation
+-- Variables are numbers: 0 = nearest binder, 1 = next, etc.
+inductive DBTerm where
+  | var (idx : Nat)    -- variable reference by binding distance
+  | lam (body : DBTerm) -- λ. body  (no name needed!)
+  | app (f arg : DBTerm)
+deriving Repr
+
+-- λ. λ. 1  (= λx. λy. x  in named form)
+def kDB : DBTerm := DBTerm.lam (DBTerm.lam (DBTerm.var 1))
+
+-- λ. 0  (= λx. x  in named form)
+def idDB : DBTerm := DBTerm.lam (DBTerm.var 0)
+
+-- Encode λf. λx. f (f x)  (Church 2)
+def twoDB : DBTerm :=
+  sorry
+
+-- 8.3 — Locally nameless
+-- Free variables are strings, bound variables are de Bruijn indices
+-- (You don't need to implement this fully — just understand the idea)
+inductive LNTerm where
+  | fvar (x : String)   -- free variable
+  | bvar (idx : Nat)    -- bound variable (de Bruijn)
+  | lam (body : LNTerm) -- binder
+  | app (f arg : LNTerm)
+deriving Repr
+#+end_src
+
+*** Key insight for PL semantics
+:PROPERTIES:
+:CUSTOM_ID: key-insight-for-pl-semantics
+:END:
+When we encode *typing contexts* =Γ = x₁:τ₁, x₂:τ₂, ...=, de Bruijn indices
+give us "index into the context" for free. The last binding is index 0, the
+second-last is index 1, etc. This makes the typing rules elegant in Lean ---
+no name-clash avoidance needed.
+
+--------------
+
+← [[../tutorial-01-basics/07-dependent-types.org][Tutorial 1 --- Unit 7]] · Next: [[file:09-substitution.org][Unit 9 --- Substitution]]