In the set A = {1, 2, {1}, {3}}, the elements in a are 1, 2, {1}, {3}, so it can be said that 1 belongs to a, and 2 belongs to a.

And {1, 2} is contained in A but does not belong to A;

The concept of set should distinguish between inclusion and attribution, the relationship between elements and sets, and the relationship between sets.