docs: def SLICE(array, n, position=None):
      """
      Returns a subset of an array.
      See https://docs.mongodb.com/manual/reference/operator/aggregation/slice/
      for more details
      :param array: Any valid expression as long as it resolves to an array.
      :param n: Any valid expression as long as it resolves to an integer.
      :param position: Optional. Any valid expression as long as it resolves to an integer.
      :return: Aggregation operator
      """
      return {'$slice': [array, position, n]} if position is not None else {'$slice': [array, n]}