This category includes core sets, Java classes, and and user-defined explicit sets. These sets are called fundamental because they can be expressed without resorting to set operations on other sets, e.g.(set ``foo'' 3.14159265 real (set 1 2 3)) which is explicit, <java.lang.String>, which is a Java class, or (union (set 1 2 3) (set 4 5 6)) which simplifies to (set 1 2 3 4 5 6).