In mathematics, an empty set refers to a set that does not contain any elements.

This battleship has enhanced antiaircraft capabilities.

This area has a strong air defense system.

He is a true visionary, always coming up with new ideas.

I enjoyed a picnic under the clear sky.

His promise was empty words.

I recorded music with my new sound recorder.

Kouka is my best friend, always a bright and energetic girl.

When he opened the door, a metallic sound echoed.

The doctor said there is a problem with his jejunum.