Exercise 4.16: In this exercise we implement the
method just described for interpreting internal definitions. We assume that
the evaluator supports let (see Exercise 4.6).
Change lookup-variable-value (4.1.3) to signal an error if
the value it finds is the symbol *unassigned*.
Write a procedure scan-out-defines that takes a procedure body and
returns an equivalent one that has no internal definitions, by making the
transformation described above.
Install scan-out-defines in the interpreter, either in
make-procedure or in procedure-body (see 4.1.3).
Which place is better? Why?
练习 4.16：在本练习中，我们实现刚才描述的解释内部定义的方法。我们假设求值器支持 let（参见练习 4.6）。
修改 lookup-variable-value（4.1.3），使其在找到的值是符号 *unassigned* 时发出错误信号。
写一个过程 scan-out-defines，它接受一个过程体并返回等价的过程体，其中不含内部定义，方法是进行上述变换。
将 scan-out-defines 安装到解释器中，安装在 make-procedure 或 procedure-body 中均可（参见 4.1.3）。哪个位置更好？为什么？