Question: can a set be an element of another set

Answer: Yes, In mathematics, a set A is a subset of a set B, or equivalently B is a superset of A, if A is ``contained'' inside B, that is, all elements of A are also elements of B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment. A is a subset of B may also be expressed as B includes A; or A is included in B.