--- a/templates/results.html	Mon Feb 08 01:20:57 2016 +0800
+++ b/templates/results.html	Wed Apr 06 11:10:23 2016 +0800
@@ -63,6 +63,7 @@
           +<a href="{{ setmarks(add=mark) }}" title="{{ desc }}">{{ mark }}</a>
         {% end %}
         *<a href="{{ setmarks() }}">all</a>
+        *<a href="{{ setmarks('') }}">none</a>
       </p>
     </header>
     <table>

--- a/viewer.py	Mon Feb 08 01:20:57 2016 +0800
+++ b/viewer.py	Wed Apr 06 11:10:23 2016 +0800
@@ -73,8 +73,8 @@
         return 'W/"{},{}"'.format(dbmt, repomt)
 
     def getmarks(self):
-        chosen = self.get_argument('marks', '')
-        if chosen:
+        chosen = self.get_argument('marks', None)
+        if chosen is not None:
             chosen = chosen.split(',')
             marks = OrderedDict((mark, MARKS[mark]) for mark in chosen if mark in MARKS)
             moremarks = OrderedDict((mark, desc) for mark, desc in MARKS.items() if mark not in chosen)
@@ -208,7 +208,7 @@
         def fn(value=None, add=None, remove=None):
             kwargs = self.request.arguments.copy()
             if add is not None:
-                kwargs['marks'] = kwargs['marks'] + [add]
+                kwargs['marks'] = marks.keys() + [add]
             elif remove is not None:
                 kwargs['marks'] = [mark for mark in marks if mark != remove]
             elif value is not None: