从mql4的文档里看到的，我没有实验过：
Global variables of the client terminal should not be mixed up with variables declared in the global scope of the MQL4 program.
Global variables are kept in the client terminal within 4 weeks since the last access, then they will be deleted automatically. An access to a global variable is not only setting of a new value, but reading of the global variable value, as well.
Global variables of the client terminal are accessible simultaneously from all MQL4 programs launched in the client terminal.
这里把Global variables跟程序里的全局变量区分开来，提到它是存放在客户端的。
