xz is a valid value for tar/untar's compression attribute.

manual/Tasks/tar.html

@@ -142,7 +142,7 @@ or "bzip2".</p>
   <tr>
     <td valign="top">compression</td>
     <td valign="top">compression method.  Allowable values are 
-       &quot;none&quot;, &quot;gzip&quot; and &quot;bzip2&quot;.  Default is
+       &quot;none&quot;, &quot;gzip&quot;, &quot;xz&quot; and &quot;bzip2&quot;.  Default is
        &quot;none&quot;.</td>
     <td valign="top" align="center">No</td>
   </tr>

manual/Tasks/unzip.html

@@ -94,7 +94,7 @@ archive.</p>
     <td valign="top"><b>Note:</b> This attribute is only available for
     the <code>untar</code> task.<br>
     compression method.  Allowable values are &quot;none&quot;,
-    &quot;gzip&quot; and &quot;bzip2&quot;.  Default is
+    &quot;gzip&quot;, &quot;xz&quot; and &quot;bzip2&quot;.  Default is
     &quot;none&quot;.</td>
     <td valign="top" align="center">No</td>
   </tr>