Add 'nixpkgs/' from commit 'abf060725d7614bd3b9f96764262dfbc2f9c2199'

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diff --git a/nixpkgs/nixos/doc/manual/administration/rollback.xml b/nixpkgs/nixos/doc/manual/administration/rollback.xml
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+<section xmlns="http://docbook.org/ns/docbook"
+        xmlns:xlink="http://www.w3.org/1999/xlink"
+        xmlns:xi="http://www.w3.org/2001/XInclude"
+        version="5.0"
+        xml:id="sec-rollback">
+ <title>Rolling Back Configuration Changes</title>
+
+ <para>
+  After running <command>nixos-rebuild</command> to switch to a new
+  configuration, you may find that the new configuration doesn’t work very
+  well. In that case, there are several ways to return to a previous
+  configuration.
+ </para>
+
+ <para>
+  First, the GRUB boot manager allows you to boot into any previous
+  configuration that hasn’t been garbage-collected. These configurations can
+  be found under the GRUB submenu “NixOS - All configurations”. This is
+  especially useful if the new configuration fails to boot. After the system
+  has booted, you can make the selected configuration the default for
+  subsequent boots:
+<screen>
+# /run/current-system/bin/switch-to-configuration boot</screen>
+ </para>
+
+ <para>
+  Second, you can switch to the previous configuration in a running system:
+<screen>
+# nixos-rebuild switch --rollback</screen>
+  This is equivalent to running:
+<screen>
+# /nix/var/nix/profiles/system-<replaceable>N</replaceable>-link/bin/switch-to-configuration switch</screen>
+  where <replaceable>N</replaceable> is the number of the NixOS system
+  configuration. To get a list of the available configurations, do:
+<screen>
+$ ls -l /nix/var/nix/profiles/system-*-link
+<replaceable>...</replaceable>
+lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055
+</screen>
+ </para>
+</section>