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+NIP-xx
+======
+
+References to previous events
+-----------------------------
+
+`draft` `optional` `author:milahu`
+
+Events contain references to previous events.
+
+<!-- "can"? Events CAN contain references to previous events. -->
+
+Implementation
+--------------
+
+Each reference to a previous events consists of
+
+- the distance between this event and the previous event, as an integer number, greater-or-equal than 1
+- the ID of the previous event
+
+The choice of distances is arbitrary.
+Some choices:
+
+- exponentially increasing distances, for example: 1, 10, 100, 1000, 10000, ...
+- linearly increasing distances, for example: 1, 10, 20, 30, 40, 50, ...
+- random distances, for example: 1, 15, 32, 58, 93, 139, 185, ...
+
+```json
+{
+  "id": ...,
+  "kind": ...,
+  ...,
+  ...,
+  "chain": [
+    <previous event distance>, <previous event ID>,
+    <previous event distance>, <previous event ID>,
+    ...
+  ]
+}
+```
+
+Soft chain of events
+--------------------
+
+This is useful to build a "soft chain" of events, so the recipient can detect missing events.
+
+Events can go missing because of
+
+- political reasons (censorship)
+- technical reasons (data loss)
+
+Redundancy
+----------
+
+Each event can contain multiple references to previous events.
+
+The next-previous event (distance 1) SHOULD always be referenced.
+
+The number of referenced previous events is arbitrary.
+It can be as low as one previous event, or as high as all previous events.
+
+This gives a tolerance for missing events:
+When a previous event is missing, the recipient can seek back to another previous event.
+
+Forward search for missing events
+---------------------------------
+
+When an event is missing, and cannot be found on any other relay, the recipient will want to "seek forward" from another previous event.
+
+Example:
+The last received event `E0` contains references to previous events `[E1, E3, E7]` with the distances `[1, 3, 7]`.
+`E1` is missing (cannot be found on any relay).
+`E3` is found and received.
+
+Problem:
+How can we find `E2`?
+Usually we would use the previous-event-reference with distance 1 of `E1`, but we dont have `E1`.
+
+Solution:
+We ask relays for events with references to `E3` or `E7` (or any other previous event in this chain).
+So relays SHOULD implement a "lookup of events by reference-to-previous-events".
+
+Limitations
+-----------
+
+Missing events can be detected only for past events, relative to a received event.
+
+Adoption
+--------
+
+Events created before the adoption of this NIP can be referenced in future events.
+To improve space-efficiency, it can make sense to distribute these back-references across multiple events ("sparse chains", "fragmented chains").
+
+TODO: rename "chain"
+--------------------
+
+```json
+{
+  ...,
+  "chain": [
+    ...
+  ]
+}
+```
+
+Rename the key "chain" to ...
+
+- previous_events
+- prevs
+- history
+- soft_chain
+
+Keywords
+--------
+
+- blockchain
+- censorship-resistance
+- tamper-proof
+- append-only