Task: Explanation
Input (English): Sadko (Russian: Садко , the name of the main character) is an opera in seven scenes by Nikolai Rimsky-Korsakov. The libretto was written by the composer, with assistance from Vladimir Belsky, Vladimir Stasov, and others. Rimsky-Korsakov was first inspired by the Bylina of Sadko in 1867, when he completed a tone poem on the subject, his Op. 5. After finishing his second revision of this work in 1891, he decided to turn it into a dramatic work. Question: Sadko has written and Cardillac by Hindemith is an example of operas written in which style? Answer: libretto
Output (English):  The libretto was written by the composer, with assistance from Vladimir Belsky, Vladimir Stasov, and others.