Sadko has written and Cardillac by Hindemith is an example of operas written in which style?
Sadko (opera):  The libretto was written by the composer, with assistance from Vladimir Belsky, Vladimir Stasov, and others. Cardillac:  wrote the libretto based on characters from the short story "Das Fräulein von Scuderi" by E.T.A. Hoffmann.
libretto