bad update on my part. Merge Ralf's work in.

Author: rouilj

test/db_test_base.py

@@ -2015,6 +2015,8 @@ def testFilteringLinkExpression(self):
             ae(filt(None, {a: ['1','-1']}), ['1','3','4'])
             ae(filt(None, {a: ['1','-1']}, ('+',a)), ['3','4','1'])
             ae(filt(None, {a: ['2','-1']}, ('+',a)), ['3','4','2'])
+            ae(filt(None, {a: ['2','-1','-4']}, ('+',a)), ['3','4','2'])
+            ae(filt(None, {a: ['2','-1','-4','-2']}, ('+',a)), ['1'])
             ae(filt(None, {a: ['1','-2']}), ['2','3','4'])
             ae(filt(None, {a: ['1','-2']}, ('+',a)), ['3','4','2'])
             ae(filt(None, {a: ['-1','-2']}, ('+',a)), ['1','2'])