Or another example is types of fingers. The empty set is a subset of every set, including the empty set itself. Likewise other elements of the set can be similarly denoted.
For infinite sets, all we can say is that the order is infinite. Same with B and b, and C and c. And if something is not in a set use. This doesn't seem very proper, does it?
We call this the universal set. How many of the students in the seminar group enjoy studying both Microeconomics and Macroeconomics?