上下文是简单的变量和表达式所需的函数，将返回一个值，并且赋值给标识，包括在符号表中的。
A context is simply the variables and functions that are needed by the expression to return a value, and are assigned to the identified included inside the Symbol Table.
当表达式执行，它从符号表中请求所需的值。
When the expression is executing, it requests from the symbol table the values needed.
当表达式执行，它从符号表中请求所需的值。
When the expression is executing, it requests from the symbol table the values needed.
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