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+# Branch Strategy / ブランチ戦略
+
+## English
+
+### Branch Overview
+
+This repository maintains a fork of the upstream flexmark-java project with the following branch structure:
+
+- **`master`**: Internal development branch
+  - Contains internal deployment configurations and customizations
+  - Used for internal features and modifications
+  - **NEVER create PRs from this branch to the upstream repository**
+
+- **`upstream/master`**: Upstream tracking branch
+  - Maintains exact synchronization with the upstream (fork source) repository
+  - **Direct pushes are PROHIBITED**
+  - **Creating/merging PRs within this repository is PROHIBITED**
+  - **ONLY permitted operation**: Pulling changes from the upstream repository
+
+### Workflow
+
+#### 1. Initial Setup
+
+Set up the upstream remote (one-time setup):
+
+```bash
+# Add upstream remote
+git remote add upstream git@github.com:vsch/flexmark-java.git
+
+# Verify remotes
+git remote -v
+# origin    git@github.com:nulab/flexmark-java.git (fetch)
+# origin    git@github.com:nulab/flexmark-java.git (push)
+# upstream  git@github.com:vsch/flexmark-java.git (fetch)
+# upstream  git@github.com:vsch/flexmark-java.git (push)
+
+# Fetch upstream branches
+git fetch upstream
+```
+
+#### 2. For Internal-Only Modifications
+
+For changes that will NOT be contributed back to upstream:
+
+```bash
+# Create feature branch from master
+git checkout master
+git pull origin master
+git checkout -b feature/internal-feature
+
+# Make changes and commit
+git add .
+git commit -m "Add internal feature"
+
+# Push and create PR to master
+git push -u origin feature/internal-feature
+# Then create PR: feature/internal-feature → master on GitHub
+```
+
+#### 3. For Upstream Contributions
+
+For changes that WILL be contributed back to upstream:
+
+**Step 3-1: Create feature branch and PR to master (internal review)**
+
+```bash
+# Create feature branch from upstream/master
+git checkout upstream/master
+git pull upstream master
+git checkout -b feature/upstream-contribution
+
+# Make changes and commit
+git add .
+git commit -m "Fix: description of the fix"
+
+# Push to your fork
+git push -u origin feature/upstream-contribution
+# Create PR: feature/upstream-contribution → master on GitHub (for internal review)
+```
+
+**Step 3-2: Create PR to upstream repository (if contribution needed)**
+
+```bash
+# After internal review approval, create PR to upstream
+# On GitHub, create PR: nulab/flexmark-java:feature/upstream-contribution → vsch/flexmark-java:master
+```
+
+**Step 3-3: After upstream PR is merged**
+
+```bash
+# Update upstream/master from upstream repository
+git checkout upstream/master
+git pull upstream master
+
+# Verify the merge
+git log --oneline -5
+
+# Push updated upstream/master to origin
+git push origin upstream/master
+```
+
+**Step 3-4: Sync changes to master**
+
+```bash
+# Create sync branch from updated upstream/master
+git checkout upstream/master
+git checkout -b sync/upstream-to-master
+
+# Push and create PR to master
+git push -u origin sync/upstream-to-master
+# Then create PR: sync/upstream-to-master → master on GitHub
+```
+
+#### 4. Regular Upstream Synchronization
+
+Keep `upstream/master` up-to-date with upstream repository:
+
+```bash
+# Update upstream/master
+git checkout upstream/master
+git pull upstream master
+git push origin upstream/master
+
+# If there are updates, sync to master
+git checkout -b sync/upstream-$(date +%Y%m%d)
+git push -u origin sync/upstream-$(date +%Y%m%d)
+# Create PR: sync/upstream-YYYYMMDD → master on GitHub
+```
+
+### Important Rules
+
+1. ✅ **Permitted Operations**
+   - Create PRs from feature branches to `master`
+   - Create PRs directly to upstream repository
+   - Create PRs from `upstream/master` to `master`
+   - Pull changes from upstream to `upstream/master`
+
+2. ❌ **PROHIBITED Operations**
+   - Creating PRs from `master` to upstream repository
+   - Direct pushes to `upstream/master`
+   - Creating/merging PRs targeting `upstream/master` within this repository
+   - Any modifications to `upstream/master` except pulling from upstream
+
+### Branch Protection Recommendations
+
+Configure the following branch protection rules on GitHub:
+
+**For `upstream/master` branch:**
+- Require pull request reviews before merging: **OFF** (PRs should not be created)
+- Require status checks to pass: **OFF**
+- Restrict who can push: **Enable** (no one should push directly)
+- Lock branch: **Consider enabling** (to prevent accidental modifications)
+
+**For `master` branch:**
+- Require pull request reviews before merging: **ON**
+- Require status checks to pass: **ON** (if CI is configured)
+- Dismiss stale pull request approvals: **ON**
+
+---
+
+## 日本語
+
+### ブランチ概要
+
+このリポジトリはupstream（flexmark-java）のフォークであり、以下のブランチ構成で運用します：
+
+- **`master`**: 社内開発用ブランチ
+  - 社内のデプロイ設定やカスタマイズを含む
+  - 社内専用の機能追加や修正に使用
+  - **このブランチからupstreamへのPRは絶対に作成しない**
+
+- **`upstream/master`**: upstream追従ブランチ
+  - Fork元（upstream）と完全に同期を保つ
+  - **直接プッシュ禁止**
+  - **このリポジトリ内でのPR作成・マージ禁止**
+  - **許可される操作**: upstreamからのpullのみ
+
+### ワークフロー
+
+#### 1. 初期セットアップ
+
+upstreamリモートの設定（初回のみ）：
+
+```bash
+# upstreamリモートを追加
+git remote add upstream git@github.com:vsch/flexmark-java.git
+
+# リモートの確認
+git remote -v
+# origin    git@github.com:nulab/flexmark-java.git (fetch)
+# origin    git@github.com:nulab/flexmark-java.git (push)
+# upstream  git@github.com:vsch/flexmark-java.git (fetch)
+# upstream  git@github.com:vsch/flexmark-java.git (push)
+
+# upstreamのブランチをフェッチ
+git fetch upstream
+```
+
+#### 2. 社内専用の修正を行う場合
+
+upstreamに出さない修正の場合：
+
+```bash
+# masterから機能ブランチを作成
+git checkout master
+git pull origin master
+git checkout -b feature/internal-feature
+
+# 変更をコミット
+git add .
+git commit -m "社内機能の追加"
+
+# プッシュしてmasterへPR作成
+git push -u origin feature/internal-feature
+# GitHub上で PR作成: feature/internal-feature → master
+```
+
+#### 3. upstreamに貢献する修正を行う場合
+
+upstreamに出す修正の場合：
+
+**手順3-1: 機能ブランチを作成してmasterへPR（社内レビュー）**
+
+```bash
+# upstream/masterから機能ブランチを作成
+git checkout upstream/master
+git pull upstream master
+git checkout -b feature/upstream-contribution
+
+# 変更をコミット
+git add .
+git commit -m "Fix: 修正内容の説明"
+
+# 自分のforkにプッシュ
+git push -u origin feature/upstream-contribution
+# GitHub上で PR作成: feature/upstream-contribution → master（社内レビュー用）
+```
+
+**手順3-2: upstreamリポジトリへPRを作成（貢献が必要な場合）**
+
+```bash
+# 社内レビュー承認後、upstreamへのPRを作成
+# GitHub上で PR作成: nulab/flexmark-java:feature/upstream-contribution → vsch/flexmark-java:master
+```
+
+**手順3-3: upstreamでPRがマージされた後**
+
+```bash
+# upstream/masterをupstreamリポジトリから更新
+git checkout upstream/master
+git pull upstream master
+
+# マージされたことを確認
+git log --oneline -5
+
+# 更新したupstream/masterをoriginにプッシュ
+git push origin upstream/master
+```
+
+**手順3-4: 変更をmasterに同期**
+
+```bash
+# 更新されたupstream/masterから同期ブランチを作成
+git checkout upstream/master
+git checkout -b sync/upstream-to-master
+
+# プッシュしてmasterへPR作成
+git push -u origin sync/upstream-to-master
+# GitHub上で PR作成: sync/upstream-to-master → master
+```
+
+#### 4. 定期的なupstream同期
+
+`upstream/master`をupstreamリポジトリと同期：
+
+```bash
+# upstream/masterを更新
+git checkout upstream/master
+git pull upstream master
+git push origin upstream/master
+
+# 更新がある場合、masterに同期
+git checkout -b sync/upstream-$(date +%Y%m%d)
+git push -u origin sync/upstream-$(date +%Y%m%d)
+# GitHub上で PR作成: sync/upstream-YYYYMMDD → master
+```
+
+### 重要なルール
+
+1. ✅ **許可される操作**
+   - 機能ブランチから`master`へのPR作成
+   - upstreamリポジトリへの直接PR作成
+   - `upstream/master`から`master`へのPR作成
+   - upstreamから`upstream/master`への変更のpull
+
+2. ❌ **禁止される操作**
+   - `master`からupstreamリポジトリへのPR作成
+   - `upstream/master`への直接プッシュ
+   - このリポジトリ内での`upstream/master`を対象とするPR作成・マージ
+   - upstreamからのpull以外での`upstream/master`の変更
+
+### ブランチ保護設定の推奨
+
+GitHub上で以下のブランチ保護ルールを設定することを推奨します：
+
+**`upstream/master`ブランチ:**
+- マージ前にプルリクエストレビューを必須: **OFF**（PRは作成されないため）
+- ステータスチェックの合格を必須: **OFF**
+- プッシュ可能なユーザーを制限: **ON**（誰も直接プッシュできないように）
+- ブランチをロック: **検討する**（誤った変更を防ぐため）
+
+**`master`ブランチ:**
+- マージ前にプルリクエストレビューを必須: **ON**
+- ステータスチェックの合格を必須: **ON**（CIが設定されている場合）
+- 古いプルリクエストの承認を却下: **ON**
+
+---
+
+## Diagram / 図解
+
+```
+┌────────────────────────────────────────────────────────────┐
+│         Upstream Repository (vsch/flexmark-java)           │
+└────────────────────────────────────────────────────────────┘
+         ↑                              │
+         │                              │ pull (after upstream merge)
+        PR(when contributing)           ↓
+         |                      upstream/master ────PR───→ master
+         │                                                    ↑
+         │                                                    │
+         │                                                    |
+    feature/xxx ───────PR─────────────────────────────────────┘
+```
+
+### Flow Summary / フロー概要
+
+```
+Internal Only:
+  master → feature/xxx → PR → master
+
+Upstream Contribution:
+  1. upstream/master → feature/xxx → PR → master (internal review)
+  2. feature/xxx → PR → vsch/flexmark-java:master (if contribution needed)
+  3. (after upstream merge) vsch/flexmark-java:master → pull → upstream/master
+  4. upstream/master → sync/xxx → PR → master
+```
+
+---
+
+## Troubleshooting / トラブルシューティング
+
+### Accidentally committed to upstream/master / 誤ってupstream/masterにコミットしてしまった場合
+
+```bash
+# Reset to match upstream (WARNING: discards local commits)
+git checkout upstream/master
+git fetch upstream
+git reset --hard upstream/master
+git push -f origin upstream/master
+```
+
+### Conflict when syncing upstream to master / upstreamからmasterへの同期で競合が発生した場合
+
+```bash
+# On sync branch
+git checkout sync/upstream-to-master
+git merge master
+# Resolve conflicts
+git add .
+git commit -m "Merge master and resolve conflicts"
+git push origin sync/upstream-to-master
+```
+
+### Lost track of upstream/master / upstream/masterの追跡を見失った場合
+
+```bash
+# Re-create upstream/master branch
+git fetch upstream
+git branch -D upstream/master
+git checkout -b upstream/master upstream/master
+git push -f origin upstream/master
+```
diff --git a/README.md b/README.md
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 ![Flexmark Icon Logo](assets/images/flexmark-icon-logo.png) flexmark-java
 =========================================================================
 
+:warning: **Branch Strategy**: This is a fork of the upstream flexmark-java project. Please read [BRANCH.md](BRANCH.md) for our branch management strategy and contribution workflow.
+
+---
+
 **flexmark-java** is a Java implementation of **[CommonMark (spec 0.28)]** parser using the
 blocks first, inlines after Markdown parsing architecture.