On a Debian system, the GNU General Public License v3 can be found in
/usr/share/common-licenses/GPL-3
+
+
+Formalities
+-----------
+
+The licence is generally indicated in each file with an SPDX
+delcaration; as is conventional, this should be read as a licence
+grant.
+
+Contributions to Hippotat are accepted based on the git commit
+Signed-off-by convention, by which the contributors' certify their
+contributions according to the Developer Certificate of Origin version
+1.1 - see the file DEVELOPER-CERTIFICATE.
+
+If you create a new file please be sure to add an appropriate licence
+header, probably something like this:
+// Copyright 2017-2022 Ian Jackson and contributors to Hippotat
+// SPDX-License-Identifier: AGPL-3.0-or-later
+// There is NO WARRANTY.