docs: def isOpen(self):
          """ Return true if this filter is not trivially false, i.e. self-contradictory.
          """
          equals = {}
          nequals = {}
          
          for test in self.tests:
              if test.op == '=':
                  if equals.has_key(test.property) and test.value != equals[test.property]:
                      # we've already stated that this arg must equal something else
                      return False
                      
                  if nequals.has_key(test.property) and test.value in nequals[test.property]:
                      # we've already stated that this arg must not equal its current value
                      return False
                      
                  equals[test.property] = test.value
          
              if test.op == '!=':
                  if equals.has_key(test.property) and test.value == equals[test.property]:
                      # we've already stated that this arg must equal its current value
                      return False
                      
                  if not nequals.has_key(test.property):
                      nequals[test.property] = set()
  
                  nequals[test.property].add(test.value)
          
          return True