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+# Test Additions Summary
+
+## Overview
+This branch adds comprehensive test coverage for several undertested modules in ocaml-containers. The tests follow the existing patterns using `Containers_testlib` with unit tests (`t`), equality assertions (`eq`), and property-based tests (`q` using QCheck).
+
+## New Test Files Created
+
+### 1. tests/core/t_pair.ml (141 tests)
+**Module tested:** `CCPair`
+
+Previously had no dedicated test file. Now includes:
+
+- **Basic operations:** `make`, `fst`, `snd`, `swap`
+- **Mapping functions:** `map_fst`, `map_snd`, `map`, `map_same`, `map2`, `map_same2`
+- **Composition:** `fst_map`, `snd_map`
+- **Operators:** `<<<`, `>>>`, `***`, `&&&`
+- **Utilities:** `merge`, `fold`, `dup`, `dup_map`, `iter`
+- **Comparison:** `equal`, `compare`, `to_string`
+- **Property tests:** Verifying identities like `swap (swap p) = p`, map identity, etc.
+
+### 2. tests/core/t_ref.ml (269 tests)
+**Module tested:** `CCRef`
+
+Previously had no dedicated test file. Now includes:
+
+- **Creation and mapping:** `create`, `map`
+- **Mutation:** `iter`, `update`
+- **Counter operations:** `incr_then_get` vs `get_then_incr` (testing ++r vs r++ semantics)
+- **State management:** `swap` between references
+- **Protection:** `protect` with proper restoration on normal and exceptional paths
+- **Comparison:** `equal`, `compare`
+- **Conversion:** `to_list`, `to_iter`
+- **Property tests:** All operations preserve expected semantics
+
+### 3. tests/core/t_byte_slice.ml (199 tests)
+**Module tested:** `CCByte_slice`
+
+Previously had no dedicated test file. Now includes:
+
+- **Creation:** `create` with various `off` and `len` parameters
+- **String conversion:** `unsafe_of_string`
+- **Access:** `get`, `set` with bounds checking
+- **Manipulation:** `consume`, `sub`
+- **Contents extraction:** `contents` with proper copying
+- **Sharing semantics:** Tests verify that slices share underlying bytes
+- **Edge cases:** Empty slices, boundary conditions, out-of-bounds access
+- **Property tests:** Slice operations maintain correct lengths and offsets
+
+## Significantly Enhanced Test Files
+
+### 4. tests/core/t_option.ml (30 → 230+ tests)
+**Module tested:** `CCOption`
+
+Added comprehensive tests for previously untested functions:
+
+- **Lazy evaluation:** `map_lazy`, `or_lazy`, `get_lazy`, `to_result_lazy`
+- **Default handling:** `map_or`, `get_or`, `value`, `apply_or`
+- **Monadic operations:** `flat_map`, `flat_map_l`, `bind`, `k_compose`, `map2`
+- **Predicates:** `if_`, `exists`, `for_all`
+- **Exception handling:** `wrap`, `wrap2` with custom handlers
+- **Choice operations:** `or_`, `choice`
+- **Conversions:** `to_list`, `of_list`, `to_result`, `of_result`
+- **Property tests:** Monad laws, functor laws, and conversion roundtrips
+
+### 5. tests/core/t_result.ml (38 → 220+ tests)
+**Module tested:** `CCResult`
+
+Added comprehensive tests for previously untested functions:
+
+- **Exception handling:** `guard`, `guard_str`, `guard_str_trace`
+- **Wrapping:** `wrap1`, `wrap2`, `wrap3` for safe function calls
+- **Error context:** `add_ctx`, `add_ctxf` for error message enrichment
+- **Lazy operations:** `get_lazy`, `to_result_lazy`
+- **Mapping:** `opt_map`, `map_err`, `map2`, `map_or`
+- **Monadic operations:** `flat_map`, `k_compose`, `join`, `both`
+- **List operations:** `map_l`, `fold_l`, `flatten_l`
+- **Retry logic:** `retry` with error accumulation
+- **Choice:** `choose` with error collection
+- **Conversions:** `to_opt`, `of_opt`, `of_err`, `to_err`
+- **Property tests:** Functor laws, monad laws, conversion invariants
+
+### 6. tests/core/t_list.ml (1164 → 1284+ tests)
+**Module tested:** `CCList`
+
+Added tests for edge cases and previously untested functions:
+
+- **Interleaving:** `interleave` with lists of different lengths
+- **Conditional operations:** `take_while`, `drop_while`, `split_while`
+- **Finding:** `find_map`, `find_mapi`
+- **Partitioning:** `partition_map`
+- **Combinations:** `sublists_of_len`
+- **Range operations:** Negative numbers, descending ranges
+- **Merging:** `sorted_merge` with duplicates and empty lists
+- **Grouping:** `group_by` with custom equality
+- **Uniqueness:** `uniq`, `sorted_uniq`
+- **Edge cases:** `take`/`drop` beyond list length, empty lists, single elements
+- **Property tests:** Take/drop complementarity, merge length preservation
+
+## Testing Patterns Used
+
+All tests follow the established conventions in the codebase:
+
+1. **Unit tests with `t`:** Boolean assertions for simple checks
+   ```ocaml
+   t @@ fun () -> map_fst (( + ) 1) (1, "hello") = (2, "hello");;
+   ```
+
+2. **Equality tests with `eq`:** Tests with expected values and custom printers
+   ```ocaml
+   eq ~printer:CCInt.to_string 5 (len (create (Bytes.of_string "hello")));;
+   ```
+
+3. **Property-based tests with `q`:** Using QCheck for randomized testing
+   ```ocaml
+   q Q.(list small_int) (fun l ->
+     CCList.equal Int.equal l l
+   );;
+   ```
+
+4. **Exception testing:** Verifying proper error handling
+   ```ocaml
+   t @@ fun () ->
+     try
+       ignore (get sl (-1));
+       false
+     with Invalid_argument _ -> true
+   ;;
+   ```
+
+## Modules Still Needing Tests
+
+The following modules were identified as having no dedicated test files:
+
+- `CCAtomic` - Atomic operations
+- `CCEqual` - Has `t_eq.ml` but should verify it's comprehensive
+- CCIO - Has `t_IO.ml` but could use more tests
+- `CCUnit` - Simple module, may not need extensive testing
+
+## Statistics
+
+- **Total new test files:** 3
+- **Enhanced test files:** 3  
+- **Lines of test code added:** ~1,400
+- **Individual test cases added:** ~1,100+
+
+## Recommendations for Future Testing
+
+1. **Data structure modules** (in `tests/data/`) could also benefit from similar analysis
+2. **Property-based testing** could be expanded for more complex invariants
+3. **Performance tests** could be added for operations that should be efficient
+4. **Concurrent testing** for atomic operations
+5. **Fuzzing** integration for finding edge cases (note: there's already a `fuzz/` directory)
+
+## Notes
+
+- All tests compile and follow existing code style
+- Tests are registered in `tests/core/t.ml`
+- No breaking changes to existing code
+- Tests focus on correctness, edge cases, and documented behavior