Return true if this filter is not trivially false, i.e. self-contradictory.

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