Add note on filtering

README.md

@@ -8,6 +8,19 @@ Each exercise includes a separate `readme.txt`
 that describes things specific to how I implemented the task.
 However, those documents are in German, which is a requirement.
 
+**Note**:
+Due to copyright restrictions,
+I am not allowed to publish many of my solutions.
+The repository `ecg-prog-filtered` only includes files solely written by me.
+It is filtered with the `./filter.sh` script.
+Please **don’t** rely on the history of this repository,
+as rewriting is part of how it can exist.
+See [Licence](#Licence) for more information on the licencing.
+
+If you want access to all of my solutions,
+please write me an email (or contact me in another way),
+I’ll see what I can do.
+
 ## Usage
 
 ### Build

filter.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: GPL-3.0-or-later
+set -euo pipefail
+export FILTER_BRANCH_SQUELCH_WARNING=1
+git branch -D filtered
+git switch -c filtered
+git filter-branch -f --tree-filter "rm -f $(rg --files-without-match 'SPDX-License-Identifier: (L)?GPL-3.0-or-later' -g '/u??/**/*' | tr '\n' ' ')" HEAD
+git push -f -u filtered filtered
+git switch -