code: def minusExtras(self):
          
          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
docs: Return a new Filter that's equal to this one,
              without extra terms that don't add meaning.