def _analyze(self):
        """ works out the updates to be performed """
        if self.value is None or self.value == self.previous:
            pass
        elif self._operation == "add":
            self._additions = self.value
        elif self._operation == "remove":
            self._removals = self.value
        elif self.previous is None:
            self._assignments = self.value
        else:
            # partial update time
            self._additions = (self.value - self.previous) or None
            self._removals = (self.value - self.previous) or None
        self._analyzed = True
dataset/ETHPy150Open datastax/python-driver/cassandra/cqlengine/statements.py/SetUpdateClause._analyze/swapped operands of `-`