[PLT] term and value

Types and Programming Languages中介绍了项（term）和值（value），这有别于符号表中变量和值的映射关系。

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（1）先通过BNF引入“项”这个语法概念，
The first line (t ::=) declares that we are defining the set of terms, and that we are going to use the letter t to range over terms.——P24

（2）然后，给语法增加求值规则，定义什么是“范式”，
A term t is in normal form if no evaluation rule applies to it.——P38

（3）“值”是被预先定义好的，因为有可能某些范式不被定义为值，
The second defines a subset of terms, called values, that are possible final results of evaluation.——P34
A closed term is stuck if it is in normal form but not a value.——P41

（4）不是值的范式，可用于表示运行时错误，
In fact, normal forms that are not values play a critical role in our analysis of run-time errors.——P38
“Stuckness” gives us a simple notion of run-time error for our simple machine.——P42

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按这个思路，类型实际上是值的一种“性质”。
预先给一些值指定类型，相当于指定了“公理”，而求值规则相当于“推导规则”，
这样建立起了一套形式推演系统。

判断某个项是否具有某种类型，就相当于在项上做形式证明。