---------------
A merge is always between the current `HEAD` and one or more
-remote branch heads, and the index file must exactly match the
+commits (usually, branch head or tag), and the index file must
+exactly match the
tree of `HEAD` commit (i.e. the contents of the last commit) when
it happens. In other words, `git-diff --cached HEAD` must
report no changes.
[NOTE]
-This is a bit of lie. In certain special cases, your index are
-allowed to be different from the tree of `HEAD` commit. The most
+This is a bit of a lie. In certain special cases, your index is
+allowed to be different from the tree of the `HEAD` commit. The most
notable case is when your `HEAD` commit is already ahead of what
is being merged, in which case your index can have arbitrary
-difference from your `HEAD` commit. Otherwise, your index entries
-are allowed have differences from your `HEAD` commit that match
-the result of trivial merge (e.g. you received the same patch
-from external source to produce the same result as what you are
+differences from your `HEAD` commit. Also, your index entries
+may have differences from your `HEAD` commit that match
+the result of a trivial merge (e.g. you received the same patch
+from an external source to produce the same result as what you are
merging). For example, if a path did not exist in the common
ancestor and your head commit but exists in the tree you are
merging into your repository, and if you already happen to have