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+Trivial merge rules
+===================
+
+This document describes the outcomes of the trivial merge logic in read-tree.
+
+One-way merge
+-------------
+
+This replaces the index with a different tree, keeping the stat info
+for entries that don't change, and allowing -u to make the minimum
+required changes to the working tree to have it match.
+
+Entries marked '+' have stat information. Spaces marked '*' don't
+affect the result.
+
+ index tree result
+ -----------------------
+ * (empty) (empty)
+ (empty) tree tree
+ index+ tree tree
+ index+ index index+
+
+Two-way merge
+-------------
+
+It is permitted for the index to lack an entry; this does not prevent
+any case from applying.
+
+If the index exists, it is an error for it not to match either the old
+or the result.
+
+If multiple cases apply, the one used is listed first.
+
+A result which changes the index is an error if the index is not empty
+and not up-to-date.
+
+Entries marked '+' have stat information. Spaces marked '*' don't
+affect the result.
+
+ case index old new result
+ -------------------------------------
+ 0/2 (empty) * (empty) (empty)
+ 1/3 (empty) * new new
+ 4/5 index+ (empty) (empty) index+
+ 6/7 index+ (empty) index index+
+ 10 index+ index (empty) (empty)
+ 14/15 index+ old old index+
+ 18/19 index+ old index index+
+ 20 index+ index new new
+
+Three-way merge
+---------------
+
+It is permitted for the index to lack an entry; this does not prevent
+any case from applying.
+
+If the index exists, it is an error for it not to match either the
+head or (if the merge is trivial) the result.
+
+If multiple cases apply, the one used is listed first.
+
+A result of "no merge" means that index is left in stage 0, ancest in
+stage 1, head in stage 2, and remote in stage 3 (if any of these are
+empty, no entry is left for that stage). Otherwise, the given entry is
+left in stage 0, and there are no other entries.
+
+A result of "no merge" is an error if the index is not empty and not
+up-to-date.
+
+*empty* means that the tree must not have a directory-file conflict
+ with the entry.
+
+For multiple ancestors, a '+' means that this case applies even if
+only one ancestor or remote fits; a '^' means all of the ancestors
+must be the same.
+
+case ancest head remote result
+----------------------------------------
+1 (empty)+ (empty) (empty) (empty)
+2ALT (empty)+ *empty* remote remote
+2 (empty)^ (empty) remote no merge
+3ALT (empty)+ head *empty* head
+3 (empty)^ head (empty) no merge
+4 (empty)^ head remote no merge
+5ALT * head head head
+6 ancest+ (empty) (empty) no merge
+8 ancest^ (empty) ancest no merge
+7 ancest+ (empty) remote no merge
+10 ancest^ ancest (empty) no merge
+9 ancest+ head (empty) no merge
+16 anc1/anc2 anc1 anc2 no merge
+13 ancest+ head ancest head
+14 ancest+ ancest remote remote
+11 ancest+ head remote no merge
+
+Only #2ALT and #3ALT use *empty*, because these are the only cases
+where there can be conflicts that didn't exist before. Note that we
+allow directory-file conflicts between things in different stages
+after the trivial merge.
+
+A possible alternative for #6 is (empty), which would make it like
+#1. This is not used, due to the likelihood that it arises due to
+moving the file to multiple different locations or moving and deleting
+it in different branches.
+
+Case #1 is included for completeness, and also in case we decide to
+put on '+' markings; any path that is never mentioned at all isn't
+handled.
+
+Note that #16 is when both #13 and #14 apply; in this case, we refuse
+the trivial merge, because we can't tell from this data which is
+right. This is a case of a reverted patch (in some direction, maybe
+multiple times), and the right answer depends on looking at crossings
+of history or common ancestors of the ancestors.
+
+Note that, between #6, #7, #9, and #11, all cases not otherwise
+covered are handled in this table.
+
+For #8 and #10, there is alternative behavior, not currently
+implemented, where the result is (empty). As currently implemented,
+the automatic merge will generally give this effect.