update docs

doc/cheatsheet.mdwn

@@ -12,4 +12,5 @@ A supplement to the [[walkthrough]].
 	walkthrough/what_to_do_when_you_lose_a_repository
 	walkthrough/recover_data_from_lost+found
 	walkthrough/Internet_Archive_via_S3
+	walkthrough/powerful_file_matching
 """]]

doc/todo/exclude_files_on_a_given_remote.mdwn

@@ -14,3 +14,5 @@ thought/known to be on A.
 	git annex drop --only-on A
 
 --[[Joey]] 
+
+[[done]]

doc/walkthrough/powerful_file_matching.mdwn

@@ -0,0 +1,36 @@
+git-annex has a powerful syntax for making it act on only certian files.
+
+The simplest thing is to exclude some files, using wild cards:
+
+	git annex get --exclude '*.mp3' --exclude '*.ogg'
+
+But you can also exclude files that git-annex's [[location_tracking]]
+information indicates are present in a given repository. For example,
+if you want to populate newarchive with files, but not those already
+on oldarchive, you could do it like this:
+
+	git annex copy --not -in oldarchive --to newarchive
+
+Without the --not, --in makes it act on files that *are* in the specified
+repository. So, to remove files that are on oldarchive:
+
+	git annex drop --in oldarchive
+
+Or maybe you're curious which files have a lot of copies, and then
+also want to know which files have only one copy:
+
+	git annex find --copies 7
+	git annex find --not --copies 2
+
+The above are the simple examples of specifying what files git-annex
+should act on. But you can specify anything you can dream up by combining
+the things above, with --and --or -( and -). Those last two strange-looking
+options are parentheses, for grouping other options. You will probably
+have to escape them from your shell.
+
+Here are the mp3 files that are in either of two repositories, but have
+less than 3 copies:
+
+	git annex find --not --exclude '*.mp3' --and \
+		-\( --in usbdrive --or --in archive -\) --and \
+		--not --copies 3