diff --git a/book/source/17-versions.md b/book/source/17-versions.md
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+<!--
+SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
+SPDX-License-Identifier: CC-BY-SA-4.0
+-->
+
+(versions)=
+# Differences between OpenPGP versions
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