There is a communal letterbox in our apartment.

He wants to learn deeply about set theory.

The concept of the absolute complement plays an important role in set theory.

In math class, we learned about the concept of union.

In math class, I learned about the concept of intersection.

In math class, the teacher explained the concept of relative complement.

Set operation is one of the basic concepts in mathematics.

Sleeping in the springtime, I ended up sleeping soundly until late in the morning.

His body build is like that of a sports player.

He was suffering from a sense of guilt.