human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: Year, Tournament, Surface, Round, Winner, Score, Length (H:MM), Sets, Federer, Nadal
###Question: What were the sets when Federer had 6 and a nadal of 13?
gpt
SELECT Sets FROM table WHERE Nadal = 13 AND Federer = 6