fix(CIP-145): updates from forum discussion

CIPs/cip-145.md

@@ -34,12 +34,20 @@ below, and would result in pruning of Data Events coinciding with Time Events.
 ### Definitions
 * A Ceramic `synchronization layer` synchronizes all the events for a Ceramic stream.
 * A Ceramic `aggregation layer` converts all the events in a Ceramic stream to its corresponding tip state.
-* A `fork point` is the last common ancestor of two events `A` and `B`.
-* A `merge point` is the first common descendant of two events `A` and `B`.
+* A `branch` for an event is the set of all events transitively covered by that event. 
+* A `pruned branch` is any branch that is not covered by the tip.
+* A `fork point` for a branch is the earliest event on that branch that is not on another branch. 
+* A `merge point` for two events is an event that transitively covers both events.
 * `A` is a `covered event` if another event `B` has `A`'s CID in its `prev` field.
+    * Said differently, if event `B` has event `A`'s CID in its `prev` field, then event `B` "covers" event `A`. By
+      transitivity, event `B` also covers every event covered by event `A`.
+    * The `time` of a Data Event is the timestamp of the earliest Time Event that covers the Data Event.
 * `A` is an `uncovered event` if there is no event with `A`'s CID in its `prev` field.
-* A `diverged stream` has more than one uncovered event.
-* A `converged stream` has a single uncovered event.
+* A `dominant Data Event` is one that is not covered by another Data Event.
+* A `non-dominant Data Event` is one that is covered by another Data Event. This applies transitively through Time
+  Events.
+* A `diverged stream` has more than one dominant Data Event.
+* A `converged stream` has a single dominant Data Event.
 * An `invalid Data Event` is one that has either an invalid signature or an expired CACAO.
 * A `valid Data Event` is one that has a valid signature and a valid CACAO (if applicable).
 
@@ -51,17 +59,33 @@ branches are pruned. Since pruning branches that contain Data Events would resul
 a Data Event to contain multiple ancestor events in its `prev` field so that a new merge point Data Event can cover
 multiple events.
 
-1. If a stream is in a diverged state (see events `A` and `B` in fig. 5), we only consider branches that contain valid,
-   uncovered Data Events.
-2. For branches that contain valid, uncovered Data Events, we only consider the first Data Event on a branch after the
-   fork.
-3. The Data Event that is covered by the earliest Time Event wins (see event `A` in fig. 5). 
-4. If two uncovered Data Events are covered by Time Events at the same block height, the Data Event with the lower CID
-   wins.
-
-Using multi-prev Data Events allows us to reduce the number of uncovered events and converge the stream so that there is
-only a single uncovered event, without any data abandoned on pruned branches. The stream's converged/diverged state can
-be determined by looking at the `prev` fields of all the Data Events for that stream.
+1. If a stream is in a diverged state, each uncovered event is a candidate tip.
+2. Branches that do not contain dominant Data Events cannot be the tip.
+3. For branches that contain dominant Data Events, consider the earliest Data Event after a fork point.
+4. The branch with the earliest Data Event becomes the tip.
+5. If multiple earliest Data Events have the same time, then the Data Event with the lowest CID becomes the tip.
+
+For an example of these rules in action, see the figure below:
+![Alt text](../assets/cip-145/rules1.png)
+
+1. Based on rule (1), the stream state has 4 candidate tips, `Time 5`, `Time 6`, `Time 4`, and `Data F`. One of these
+   candidate tips will become the tip of the stream.
+   ![Alt text](../assets/cip-145/rules2.png)
+2. The stream forks between the branches for `Time 1` and `Data A`. Based on rules (3) and (4), the branch for `Data A`
+   is the only branch considered for tip selection.
+   ![Alt text](../assets/cip-145/rules3.png)
+3. This branch later forks into additional branches for `Data E`, `Time 4`, and `Data F`. Based on rules (3) and (4),
+   the branches for `Data E` and `Data F` are the only branches considered for tip selection.
+   ![Alt text](../assets/cip-145/rules4.png)
+4. Based on rule (5), since there is a Time Event for `Data E` but not for `Data F`, only the branch for `Data E` is
+   considered for tip selection.
+   ![Alt text](../assets/cip-145/rules5.png)
+5. `Time 6` is the tip of the surviving branch, and therefore becomes the tip of the stream.
+   ![Alt text](../assets/cip-145/rules6.png)
+
+Using multi-prev Data Events allows us to reduce the number of dominant Data Events and converge the stream so that
+there is only a single dominant Data Event, without any data abandoned on pruned branches. The stream's
+converged/diverged state can be determined by looking at the `prev` fields of all the Data Events for that stream.
 
 Events that have invalid signatures cannot be tips, even if uncovered. This has important implications for Data Events
 with expired CACAOs. In figures 5 and 6, if we assume event `A` has an expired CACAO, the aggregation layer can choose