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So far we've learned about elementary branches for maintaining changes apart from a primary development branch and development branches for coordinating asynchronous work on a single project (see Elementary Branches -- Maintaining Private Changes and Development Branches -- The star-merge Style of Cooperation).
In this chapter, we'll briefly describe a third kind of branch that's useful when a project consists of multiple "forks" -- multiple, equally primary branches.
Let's suppose, somewhat abstractly, that Alice and Bob's mainline has grown quite large:
        mainline
        --------
        base-0
        patch-1
        ....
        patch-23
        patch-24
        patch-25
        ...
        patch-42
At some point, perhaps because some controversy has emerged over
choices made in the mainline
, a new developer, Derick, declares 
a fork and starts his own branch:
        mainline                derick
        --------                ------
        base-0          ------> base-0
        patch-1        '
        ....          '
        patch-23 ----'
        patch-24
        patch-25
        ...
        patch-42
We already know that Derick can use update
 or replay
 to keep
current with the mainline, but what he doesn't want to?   What if
Derick wants the changes in patch-25
 and patch-42
, but none of the
other post-patch-23
 changes from the mainline
?
Derick can apply specific changes from the mainline
 by specifying the
exact revision he wants, rather than just specifying a version:
        % cd ~/wd
        % tla get hello-world--derick--0.1 derick
        % cd derick
        % tla replay -A lord@emf.net--2003-example \
                 hello-world--mainline--0.1--patch-23
        % tla replay -A lord@emf.net--2003-example \
                 hello-world--mainline--0.1--patch-42
        % tla missing -A lord@emf.net--2003-example \
                 hello-world--mainline--0.1
        patch-24
        patch-25
        ...
        patch-41
        % tla logs -A lord@emf.net--2003-example \
                 hello-world--mainline--0.1
        base-0
        patch-1
        ...
        patch-22
        patch-23
        patch-42
Cherrypicking
 changes in this manner isn't necessarily easy or even
practical.  It depends, for example, on the mainline
 changes being
"clean changesets" (see Using commit Well -- The Idea of a Clean Changeset).
Nevertheless, for some projects, especially those characterized by lots of "forks", this technique can be useful.
Learning Note: Multiple revisions may be replayed with a single
command, simply by giving all of them on the command line at once.  The
replay
 command also has a --list
 option which can useful for
cherrypicking many changes at once.  If you find yourself replaying
specific revisions often, you should take a look at the --list
 option
in tla replay --help
.
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