In the elite clubs of 1930s Granada, new members were admitted through a secret ballot using white and black balls, where a single black ball was enough to reject a candidate. Federico García Lorca began writing a play about a young man rejected by his club for being gay, but he only managed to write four pages before he was killed in 1936. Nine decades later, those lost fragments have become the heart of one of the most powerful films at this year's Cannes.