Question: "Purple Rain-Union" is the sixth episode of the fourth season of the animated comedy series "Bob's Burgers", it aired on Fox in the United States on which date, the title of the episode is a reference to the song, film, and album "Purple Rain" by Prince?
Sentence (from "Purple Rain (song)"): The song is a combination of rock, R&B, gospel, and orchestral music.
Relevant evidence: No