<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_mongo(self):
 """Translate projection to MongoDB query form. :return: Dictionary to put into a MongoDB JSON query :rtype: dict """

        d = copy.copy(self._fields)
        for k, v in self._slices.items():
            d[k] = {'$slice': v}
        return d