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+# Elisp / Emacs Rules
+
+Applies to: `**/*.el`
+
+## Style
+
+- 2-space indent, no tabs
+- Hyphen-case for identifiers: `cj/do-thing`, not `cj/doThing`
+- Naming prefixes:
+  - `cj/name` — user-facing functions and commands (bound to keys, called from init)
+  - `cj/--name` — private helpers (double-dash signals "internal")
+  - `<module>/name` — module-scoped where appropriate (e.g., `calendar-sync/parse-ics`)
+- File header: `;;; foo-config.el --- brief description -*- lexical-binding: t -*-`
+- `(provide 'foo-config)` at the bottom of every module
+- `lexical-binding: t` is mandatory — no file without it
+
+## Function Design
+
+- Keep functions under 15 lines where possible
+- One responsibility per function
+- Extract helpers instead of nesting deeply — 5+ levels of nesting is a refactor signal
+- Prefer named helpers over lambdas for anything nontrivial
+- No premature abstraction — three similar lines beats a clever macro
+
+Small functions are the single strongest defense against paren errors. Deeply nested code is where AI and humans both fail.
+
+## Requires and Loading
+
+- Every `(require 'foo)` must correspond to a loadable file on the load-path
+- Byte-compile warnings about free variables usually indicate a missing `require` or a typo in a symbol name — read them
+- Use `use-package` for external (MELPA/ELPA) packages
+- Use plain `(require 'foo-config)` for internal modules
+- For optional features, `(when (require 'foo nil t) ...)` degrades gracefully if absent
+
+## Lexical-Binding Traps
+
+- `(boundp 'x)` where `x` is a lexical variable always returns nil. Bind with `defvar` at top level if you need `boundp` to work, or use the value directly.
+- `setq` on an undeclared free variable is a warning — use `let` for locals or `defvar` for module-level state
+- Closures capture by reference. Avoid capturing mutating loop variables in nested defuns.
+
+## Regex Gotchas
+
+- `\s` is NOT whitespace in Emacs regex. Use `[ \t]` or `\\s-` (syntax class).
+- `^` in `string-match` matches after `\n` OR at position 0 — use `(= (match-beginning 0) start)` for positional checks when that matters.
+- `replace-regexp-in-string` interprets backslashes in the replacement. Pass `t t` (FIXEDCASE LITERAL) when the replacement contains literal backslashes.
+
+## Keybindings
+
+- `keymap-global-set` for global; `keymap-set KEYMAP ...` for mode-local
+- Group module-specific bindings inside the module's file
+- Autoload cookies (`;;;###autoload`) don't activate through plain `(require ...)` — use the form directly, not an autoloaded wrapper
+
+## Module Template
+
+```elisp
+;;; foo-config.el --- Foo feature configuration -*- lexical-binding: t -*-
+
+;;; Commentary:
+;; One-line description.
+
+;;; Code:
+
+;; ... code ...
+
+(provide 'foo-config)
+;;; foo-config.el ends here
+```
+
+Then `(require 'foo-config)` in `init.el` (or a config aggregator).
+
+## Editing Workflow
+
+- A PostToolUse hook runs `check-parens` and `byte-compile-file` on every `.el` save
+- If it blocks, read the error — don't retry blindly
+- Prefer Write over repeated Edits for nontrivial new code; incremental edits accumulate subtle paren mismatches