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+=========================
+Obsolete concept
+=========================
+
+
+Obsolete marker is a powerful concept that allow mercurial to safely handle
+history rewriting operations. It is a new type of relation between Mercurial
+changesets that track the result of history rewriting operations.
+
+This concept is simple to define and provides a very solid base to:
+
+
+- Very fast history rewriting operations,
+
+- auditable and reversible history rewritting process,
+
+- clean final history,
+
+- share and collaborate on mutable part of the history,
+
+- gracefully handle history rewriting conflict,
+
+- allows various history rewriting UI to collaborate with a underlying common API.
+
+
+-----------------------------------------------------
+Basic concept
+-----------------------------------------------------
+
+
+Every history rewriting operation stores the information that old rewritten
+changesets has newer version available in a set of changeset.
+
+This simple rules allows to express any possible history rewriting operation:
+
+
+
+
+.. figure:: ./figures/example-1-update.png
+
+ *Updating* a changeset
+
+ Create one obsolete marker: ``([A'] obsolete A)``
+
+
+
+.. figure:: ./figures/example-2-split.png
+
+ *Splitting* a changeset in multiple one
+
+ Create one obsolete marker ``([B1, B2] obsolete B)]``
+
+
+.. figure:: ./figures/example-3-merge.png
+
+ *Merging* multiple changeset in a single one
+
+ Create two obsolete markers ``([C] obsolete A), ([C] obsolete B)``
+
+.. figure:: ./figures/example-4-reorder.png
+
+ *Moving* changeset around
+
+ Reordering those two changesets need two obsolete markers:
+ ``([A'] obsolete A), ([B'] obsolete B)``
+
+
+
+.. figure:: ./figures/example-5-delete.png
+
+ *Removing* a changeset:
+
+ One obselete marker ``([] obsolete B)``
+
+
+To conclude, a single obsolete marker express a relation from **0..n** new
+changesets to **1** old changeset.
+
+-----------------------------------------------------
+Basic Usage
+-----------------------------------------------------
+
+Obsolete markers create a perpendicular history: **a versionned version of the
+changeset graph**. This means that we can have the same feature we have for
+versioned files but applied to changeset:
+
+First: we can display a **coherent view** of the history graph with only a
+single version of your changeset are displayed by the UI.
+
+Second, because obsolete changeset content are still **available**. You can
+
+ * **browse** the content of your obsolete commit,
+
+ * **compare** newer and older version of a changeset,
+
+ * **restore** content of previously obsolete changeset.
+
+Finally, obsolete marker can be **exchanged between repositories**. You are able to
+share the result on your history rewriting operation with other and **collaborate
+on mutable part of the history**.
+
+Conflicting history rewriting operation can be detected and **resolved** as easily
+as conflicting changes on file.
+
+
+-----------------------------------------------------
+Detecting and solving tricky situation
+-----------------------------------------------------
+
+History rewriting can lead to complex situation. Obsolete marker introduce a
+simple representation this complex reality. But people using complex workflow
+will one day or another you have to face the intrinsics complexity of some
+situation.
+
+This section describe possible situations, define precise set of changesets
+involved in such situation and explains how error case can we automatically
+resolved using available information.
+
+
+obsolete changesets
+````````````````````
+
+Old changesets left behind by obsolete operation are said **obsolete**.
+
+With current version of mercurial, this *obsolete* part is stripped from the
+repository before the end of every rewritting operation.
+
+.. figure:: ./figures/error-obsolete.png
+
+ Rebasing `B` and `C` on `A` (as `B'`, `C'`)
+
+ This rebase operation added two obsolete markers from new changesets to old
+ changesets. These Two old changesets are now part of the *obsolete* part of the
+ history.
+
+In most case the obsolete set will be fully hidden to both UI and discovery so
+user do not have to care about them unless he wants to audit history rewriting
+operation.
+
+Unstable changesets
+```````````````````
+
+While exploring obsolete marker possibility a bit further you way end up with
+*obsolete* changeset with *non-obsolete* children. There is two common ways to
+achieve this:
+
+* Pull a changeset based of an old version of a changeset [#]_.
+
+* Use a partial rewriting operation. For example amend on a changeset with
+ childrens.
+
+*Non-obsolete* changeset based on *obsolete* one are said **unstable**
+
+.. figure:: ./figures/error-unstable.png
+
+ Amend `A` into `A'` leaving `B` behind.
+
+ In this situation we can not consider `B` as *obsolete*. But we have all
+ necessary data to detect `B` as an *unstable* branch of the history because
+ its parent `A` is *obsolete*. In addition, we have enough data to
+ automatically resolve this instability: we know that the new version of `B`
+ parent (`A`) is `A'`, We can deduce that we should rebase `B` on `A'` to get
+ a stable history again.
+
+Proper warning should be issued when part of the history become unstable. UI
+will be able to use the obsolete marker to automatically suggest resolution to
+the user of even carry them out for him.
+
+
+XXX details automatic resolution for
+
+* movement
+
+* handling deletion
+
+* handling split on multiple head
+
+
+.. [#] For this to happen one needs to explicitly enable exchange of draft
+ changeset. See phase help for details.
+
+The two part of the obsolete set
+``````````````````````````````````````
+
+The previous section show that it could be two kinds of *obsolete* changeset:
+
+
+* *obsolete* changeset with no or *obsolete* only descendants, said **extinct**.
+
+* *obsolete* changeset with *unstable* descendants, said **suspended**.
+
+
+.. figure:: ./figures/error-extinct.png
+
+ Amend `A` and `C` leaving `B` behind.
+
+ In this example we have two *obsolete* changesets: `C` with no *unstable*
+ children is *extinct*. `A` with *unstable* descendant (`B`) is *suspended*.
+ `B` is *unstable* as before.
+
+
+Because nothing outside the obsolete set default on *extinct* changesets, they
+can be safely hidden in the UI and even garbage collected. *Suspended* changeset
+have to stay visible and available until they unstable descendant are rewritten
+in stable version.
+
+
+Conflicting rewriting
+``````````````````````
+
+If people start to concurrently edit the same part of the history they will
+likely meet conflicting situation when a changeset have been rewritten in two
+different versions.
+
+
+.. figure:: ./figures/error-conflicting.png
+
+ Conflicting rewriting of `A` into `A'` and `A''`
+
+This kind of conflict is easy to detect with obsolete marker because an obsolete
+changeset have more than one new version. It may be seen as the multiple heads
+case Mercurial warn you about on pull. It is resolved the same way by a merge of
+A' and A'' that will keep the same parent than `A'` and `A''` with two obsolete
+markers pointing to both `A` and `A'`
+
+.. warning:: TODO: Add a schema of the resolution. (merge A' and A'' with A as
+ ancestor and graft the result of A^)
+
+Allowing multiple new changesets to obsolete a single one allow to distinct a
+splitted changeset from history rewriting conflict.
+
+Reliable history
+``````````````````````
+
+Obsolete marker really help to smooth rewriting operation process. However they
+do not change the fact that **you should only rewrite the mutable part of the
+history**. The phase concept enforce this rules by explicitly defining a
+public immutable set of changeset. Rewriting operation refuse to work on
+public changeset, but they is still some corner case where changesets
+rewritten in the past are made public.
+
+Special rules apply for obsolete marker pointing to public changeset
+
+* Public changesets are excluded from the obsolete set (public changeset are
+ never hidden or candidate to garbage collection)
+
+* *newer* version of public changeset are said **latecomer** and highlighted as
+ error case.
+
+
+Solving such error is easy. Because we know what changeset a *latecomer* try to
+rewrite, we can easily compute a smaller changeset containing only the change
+from the old *public* to the new *latecomer*.
+
+
+.. warning:: add a schema
+