绘制符号表达式的图形 ezplot
符号表达式的求和函数 symsum
符号表达式的化简 Simplifying of symbolic expression
符号表达式的替换 Replacing of symbolic expression
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.
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