Thank you! You make things more clear.
However let me try to make a cunning explanation about "a set of methods ...".
1. In computer terminologies, a set (not a non-empty set) of something means the number of elements in the set can be 0, 1, 2, ..., n.
2. If you designe a container class which abstracts concept of a set (a collection of somethings), I am definitely sure you will define and implement a method: bool IsEmpty() -- Is the set empty?
The above is just my cunning explanation. You make things more clear.