表单 begin 有两个目的.

The form begin has two purposes.

1. To sequence the evaluation of expressions 
2. To "splice" sequences together (used by macros)

第一个是最常用的:

(begin e0 e1 ...)

将按顺序计算表达式 e0 e1 ....

will evaluate the expressions e0 e1 ... in order.

当宏扩展为多个定义和/或表达式时使用第二个.

The second is used when a macro expands to multiple definitions and/or expressions.

以下面为例

(begin
   (begin d1 e1 d2 d3)
   (begin)
   e2
   ...)

将被宏扩展器扁平化为:

will be flattened by the macro expander into:

(begin d1 e1 d2 d3 e2 ...)

现在是为什么(开始)完全允许?"的问题.如果 begin 用于目的 1(排序)，则可能不允许使用空的 begin.对于目的 2(拼接)，使用 (begin) 作为一个什么都不做的宏的结果是非常方便的.考虑一个宏 (debug expression)，它可以扩展为 expression(启用调试时)或禁用调试时扩展为 (begin).

Now to the question "Why is (begin) allowed at all?". If begin was used for purpose 1 (sequencing) then an empty begin could be disallowed. For purpose 2 (splicing) it is very convenient to use (begin) as the result of a macro that does nothing. Consider a macro (debug expression) that either expands into expression (when debugging is enabled) or into (begin) when debugging is disabled.