[PATCH] gnu: Add emacs-undo-tree.

[KAction]

From: Dmitry Bogatov <address@hidden>

  * gnu/packages/emacs.scm (emacs-undo-tree): New variable.
---
 gnu/packages/emacs.scm | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/gnu/packages/emacs.scm b/gnu/packages/emacs.scm
index 7d95f55..55717ff 100644
--- a/gnu/packages/emacs.scm
+++ b/gnu/packages/emacs.scm
@@ -1007,6 +1007,42 @@ and stored in memory.")
     (description "This package provides a modern list API library for Emacs.")
     (license license:gpl3+)))
 
+(define-public emacs-undo-tree
+  (package
+    (name "emacs-undo-tree")
+    (version "0.6.4")
+    (source (origin
+              (method git-fetch)
+              (uri (git-reference
+                    (url "http://dr-qubit.org/git/undo-tree.git";)
+                    (commit "release/0.6.4")))
+              (file-name (string-append name "-" version "-checkout"))
+              (sha256
+                (base32
+                  "0b6hnv6bq1g5np5q2yw9r9aj1cxpp14akm21br7vpb7wp01fv4b3"))))
+    (build-system emacs-build-system)
+    (home-page "http://www.dr-qubit.org/emacs.php";)
+    (synopsis "Treat undo history as a tree")
+    (description "Emacs has a powerful undo system.  Unlike the standard
+undo/redo system in most software, it allows you to recover *any* past
+state of a buffer (whereas the standard undo/redo system can lose past
+states as soon as you redo).  However, this power comes at a price: many
+people find Emacs' undo system confusing and difficult to use, spawning
+a number of packages that replace it with the less powerful but more
+intuitive undo/redo system.
+
+Both the loss of data with standard undo/redo, and the confusion of
+Emacs' undo, stem from trying to treat undo history as a linear sequence
+of changes.  It's not.  The @code{undo-tree-mode} provided by this
+package replaces Emacs' undo system with a system that treats undo
+history as what it is: a branching tree of changes.  This simple idea
+allows the more intuitive behaviour of the standard undo/redo system to
+be combined with the power of never losing any history.  An added side
+bonus is that undo history can in some cases be stored more efficiently,
+allowing more changes to accumulate before Emacs starts discarding
+history.")
+    (license license:gpl3+)))
+
 (define-public emacs-s
   (package
     (name "emacs-s")
-- 
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