The function S1 is defined outside of the current file, and because the compiler doesn't actually know where the external function is, we leave this part of the symbol table blank for now.

函数s1没有在当前文件中定义，因为编译器实际上并不知道这个外部函数在哪儿，我们现在把这个符号表项保留为空白。

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So-called empty sign means the symbol with the form of blank, partition, pause and distance etc. functioning as a special sign that can't be acted for in various sign systems.

所谓空符号，是指以空白、间隔、停顿、距离等作为符号形式，在各种符号系统中起着不可替代作用的一类特殊符号。

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