2.3 Symbolic Data

All the compound data objects we have used so far were constructed ultimately

from numbers. In this section we extend the representational capability of our

language by introducing the ability to work with arbitrary symbols as data.

我们迄今使用的所有复合数据对象，归根结底都是由数构造而成的。在本节中，我们通过引入以任意符号 (symbols) 作为数据的能力，来扩展语言的表示能力。