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+# ERT (Emacs Lisp Regression Testing)
+
+Built-in testing framework for Emacs Lisp, providing comprehensive test definition, execution, and debugging capabilities.
+
+## Quick Start
+
+### Basic Test
+
+```elisp
+(require 'ert)
+
+(ert-deftest test-addition ()
+  "Test that addition works correctly."
+  (should (= 4 (+ 2 2)))
+  (should (= 0 (+ -5 5))))
+```
+
+### Run Tests
+
+**Interactively:**
+```elisp
+M-x ert RET t RET
+```
+
+**From Command Line:**
+```bash
+emacs -batch -l ert -l my-tests.el -f ert-run-tests-batch-and-exit
+```
+
+### Assertions
+
+```elisp
+;; Assert true
+(should (= 2 (+ 1 1)))
+
+;; Assert false
+(should-not (= 3 (+ 1 1)))
+
+;; Assert error
+(should-error (/ 1 0) :type 'arith-error)
+```
+
+## Key Features
+
+- **Built-in:** No installation required (Emacs 24+)
+- **Interactive debugging:** Backtrace inspection, test reruns
+- **Flexible selectors:** Run specific tests by name, tag, or status
+- **Batch mode:** CI/CD integration with exit codes
+- **Mocking support:** Easy mocking via dynamic binding
+- **Rich assertions:** Detailed failure reporting with subexpression values
+
+## Common Patterns
+
+### Temporary Buffers
+
+```elisp
+(ert-deftest test-buffer-operation ()
+  (with-temp-buffer
+    (insert "test content")
+    (my-function)
+    (should (string= (buffer-string) "expected"))))
+```
+
+### Cleanup with unwind-protect
+
+```elisp
+(ert-deftest test-with-cleanup ()
+  (let ((temp-file (make-temp-file "test-")))
+    (unwind-protect
+        (progn
+          (write-region "data" nil temp-file)
+          (should (file-exists-p temp-file)))
+      (delete-file temp-file))))
+```
+
+### Mocking Functions
+
+```elisp
+(ert-deftest test-with-mock ()
+  (cl-letf (((symbol-function 'external-call)
+             (lambda () "mocked result")))
+    (should (string= "mocked result" (my-function)))))
+```
+
+### Test Organization with Tags
+
+```elisp
+(ert-deftest test-quick-operation ()
+  :tags '(quick unit)
+  (should (fast-function)))
+
+;; Run only quick tests
+M-x ert RET :tag quick RET
+```
+
+## Interactive Debugging Commands
+
+When viewing test results (after `M-x ert`):
+
+- `.` - Jump to test definition
+- `d` - Re-run test with debugger
+- `b` - Show backtrace
+- `r` - Re-run test
+- `R` - Re-run all tests
+- `l` - Show executed assertions
+- `m` - Show messages
+
+## Best Practices
+
+1. **Name tests descriptively:** `package-test-feature` format
+2. **One assertion focus per test:** Makes failures clear
+3. **Isolate test environment:** Use `let`, temporary buffers
+4. **Always cleanup:** Use `unwind-protect` for resources
+5. **Tag tests:** Organize by speed and type
+6. **Test error cases:** Use `should-error` for edge cases
+7. **Mock external dependencies:** Avoid filesystem/network I/O
+
+## Using the Skill
+
+This skill provides:
+- Complete API reference for all ERT functions
+- Comprehensive best practices and patterns
+- Detailed examples for common testing scenarios
+- Debugging techniques and workflow integration
+- CI/CD integration examples
+
+Consult `SKILL.md` for in-depth documentation.
+
+## Learning Path
+
+1. **Basics:** `ert-deftest`, `should`, running tests interactively
+2. **Assertions:** `should-not`, `should-error` with error types
+3. **Environment:** `with-temp-buffer`, `let`, `unwind-protect`
+4. **Organization:** Tags, test naming, selectors
+5. **Advanced:** Mocking, fixtures, custom assertions
+6. **Integration:** Batch mode, CI/CD, Makefiles
+
+## External Resources
+
+- [Official ERT Manual](https://www.gnu.org/software/emacs/manual/html_mono/ert.html)
+- [ERT in Emacs Info](info:ert) or `C-h i m ert RET`
+- [ERT Reference Card](https://github.com/fniessen/refcard-ERT)