Moreover, these extensions were all achieved using the same basic approach: recursive application of the same basic set of concepts at different levels of abstraction.
On top of that the whole thing is recursive: a set of systems is a system in his own right.
此外，所有这些都是递归的：一个系统集合完全也可以是一个系统。
3
But, you can only accomplish two other popular operations on a set of Numbers, finding the minimum and maximum, by a recursive template or extension function.