--- a/kharon.py	Mon Dec 08 13:50:08 2008 -0800
+++ b/kharon.py	Mon Dec 08 14:32:06 2008 -0800
@@ -37,6 +37,9 @@
     if popen.returncode:
         raise Exception('Process failed: %s' % repr(params))
 
+def dir_for_trans(transid):
+    return os.path.join(HADES_DIR, '%.9d' % transid)
+
 if __name__ == '__main__':
     if not (os.path.exists(HADES_DIR) and os.path.isdir(HADES_DIR)):
         shell('mkdir', HADES_DIR)
@@ -54,7 +57,28 @@
     config = Config(STATE_FILENAME)
 
     if options.undo:
-        raise NotImplementedError('Undo not implemented yet.')
+        transactions = []
+        for arg in args:
+            try:
+                transid = int(arg)
+            except ValueError:
+                print('Unknown transaction: %s' % arg)
+                sys.exit(-1)
+            transactions.append(transid)
+        if not transactions and config.nextid:
+            transactions.append(config.nextid - 1)
+        for transid in transactions:
+            dirname = dir_for_trans(transid)
+            if not os.path.exists(dirname):
+                print('Transaction %d does not exist.' % transid)
+                sys.exit(-1)
+
+        print('Sorry, undo is not implemented yet.'
+              'But you can find what you need in the following '
+              'directory/directories:\n')
+        print('\n'.join([dir_for_trans(transid)
+                         for transid in transactions]))
+        sys.exit(-1)
     else:
         files = []
         for arg in args:
@@ -70,7 +94,7 @@
             files.append(filename)
 
         thisid = config.nextid
-        basedir = os.path.join(HADES_DIR, '%.9d' % thisid)
+        basedir = dir_for_trans(thisid)
         shell('mkdir', basedir)
         config.nextid = thisid + 1
         config.save()