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+**Title:** Add checkpoint support </br>
+**Date:** 2021-12-09 </br>
+
+# Summary
+Add experimental checkpoint support.
+
+# Description
+Sigsum collaborated on a common
+	[checkpoint format](https://github.com/google/trillian-examples/tree/master/formats/log)
+a while back.  A checkpoint is basically a cosigned tree head.
+
+The current decision is to add experimental support for checkpoints.  There is
+no formal decision to adopt the above checkpoint yet, hence _experimental_.
+
+To keep it simple:
+1. Don't add any timestamp extension.
+2. Only serve the most recent tree head as a checkpoint.  This allows us to
+experiment with external feeders and distributors that are not part of the log.