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+# pdftract-1zhu: Incremental Update Chain Handler Implementation
+
+## Summary
+The `/Prev` chain handler for incremental PDF updates was already fully implemented in `crates/pdftract-core/src/parser/xref.rs`. All acceptance criteria are met.
+
+## Implementation Details
+
+### Core Function: `load_xref_with_prev_chain`
+Located at `xref.rs:2154-2269`, this function:
+1. Loads the trailing xref (auto-detects traditional vs stream vs hybrid)
+2. Recursively follows `/Prev` pointers in trailers
+3. Merges revisions with override semantics (newest wins for each object number)
+4. Returns the latest revision's trailer
+
+### Key Features Implemented
+- **Cycle detection**: `HashSet<u64>` tracks visited offsets, emits `STRUCT_CIRCULAR_REF`
+- **Depth limit**: `MAX_PREV_DEPTH = 32`, emits `STRUCT_DEPTH_EXCEEDED` on exceed
+- **Override semantics**: For each ObjRef, LATER revision (loaded first) wins
+- **Trailer handling**: Latest revision's trailer returned (newest /Root, /Info, /ID)
+- **Edge cases**:
+  - `/Prev <= 0` treated as absent (no previous revision)
+  - `/Prev > file_size` emits `STRUCT_INVALID_PREV_OFFSET`, ignores /Prev
+  - Hybrid files: each revision calls `load_single_xref` which handles hybrid merging
+
+### Hybrid Support
+`load_single_xref` (xref.rs:2071-2107) detects hybrid files via `is_hybrid_trailer` and calls `merge_hybrid` when `/XRefStm` is present. This is invoked at each level of the `/Prev` chain.
+
+## Test Results
+
+### All `/Prev` Chain Tests PASS (12/12)
+```
+test parser::xref::tests::test_prev_chain_negative_prev_is_absent ... ok
+test parser::xref::tests::test_prev_chain_object_added_only_in_latest ... ok
+test parser::xref::tests::test_prev_chain_trailer_from_latest ... ok
+test parser::xref::tests::test_prev_chain_three_revisions_latest_wins ... ok
+test parser::xref::tests::test_prev_chain_zero_prev_is_absent ... ok
+test parser::xref::tests::test_prev_chain_cycle_detection ... ok
+test parser::xref::tests::test_prev_chain_depth_limit ... ok
+test parser::xref::tests::test_prev_chain_invalid_offset ... ok
+test parser::xref::tests::test_prev_chain_object_add_modify_free ... ok
+test parser::xref::tests::test_prev_chain_hybrid_file ... ok
+test parser::xref::tests::proptest_prev_chain_tests::prop_prev_chain_random_no_panic ... ok
+test parser::xref::tests::proptest_prev_chain_tests::prop_prev_chain_random_offsets_no_panic ... ok
+```
+
+### Acceptance Criteria Status
+| Criterion | Status | Notes |
+|-----------|--------|-------|
+| 3-revision chain, latest wins | ✅ PASS | test_prev_chain_three_revisions_latest_wins |
+| Object 7: add/modify/free lifecycle | ✅ PASS | test_prev_chain_object_add_modify_free |
+| Object added only in latest | ✅ PASS | test_prev_chain_object_added_only_in_latest |
+| Trailer from latest revision | ✅ PASS | test_prev_chain_trailer_from_latest |
+| /Prev cycle detection | ✅ PASS | test_prev_chain_cycle_detection |
+| Depth limit (32 revisions) | ✅ PASS | test_prev_chain_depth_limit |
+| proptest: random configs | ✅ PASS | prop_prev_chain_random_no_panic |
+| INV-8 maintained | ✅ PASS | No changes to xref module structure |
+
+## Pre-existing Issues
+Some xref tests fail (forward_scan, multi-subsection parsing), but these are unrelated to the `/Prev` chain handler and represent pre-existing issues from earlier work.
+
+## Files
+- Implementation: `crates/pdftract-core/src/parser/xref.rs:2154-2269`
+- Tests: `crates/pdftract-core/src/parser/xref.rs:3826-4338`
+- Constants: `MAX_PREV_DEPTH = 32` at line 2113
+
+## Verification
+```bash
+cargo test -p pdftract-core prev_chain --lib
+# Result: 12 passed; 0 failed
+```