def minusExtras(self):
        """ Return a new Filter that's equal to this one,
            without extra terms that don't add meaning.
        """
        assert self.isOpen()
        
        trimmed = self.clone()
        
        equals = {}
        
        for test in trimmed.tests:
            if test.op == '=':
                equals[test.property] = test.value

        extras = []

        for (i, test) in enumerate(trimmed.tests):
            if test.op == '!=' and equals.has_key(test.property) and equals[test.property] != test.value:
                extras.append(i)

        while extras:
            trimmed.tests.pop(extras.pop())

        return trimmed
Return a new Filter that's equal to this one,
            without extra terms that don't add meaning.