An opera libretto must be more concise than a play.

His opera libretti and works of fiction were far less successful.

Or maybe I've been exposed to too many opera librettos.

You're right, you have been reading too many opera librettos.

He was the author of several plays and opera libretti.

He also wrote some music criticism and translated opera librettos.

And the slap to my face came from the same opera libretto.

A number of them were later adapted as plays or opera librettos.

He wrote many more opera librettos for the emperor's theatre.

In recent years he has also written opera libretti.