Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE table_name_15 (nat VARCHAR, name VARCHAR, transfer_window VARCHAR, type VARCHAR)
### Question:
Marcell Jansen with a transfer window of summer, and of transfer as type is from what nation?
### Answer:
SELECT nat FROM table_name_15 WHERE transfer_window = "summer" AND type = "transfer" AND name = "marcell jansen"