try-except
这里展示了 python 的报错机制
相比于正常的“被动报错”，这里可以返回遇到具体错误时其他的出题情况，避免一些 bug
Programming Languages
A computer typically executes programs written in many different programming languages.
Machine languages: statements are interpreted by the hardware itself
- A fixed set of instructions invoke operations implemented by the circuitry of the central processing unit (CPU)
- Operations refer to specific hardware memory addresses; no abstraction mechanisms
High-level languages: statements & expressions are interpreted by another program or compiled (translated) into another language
- Provide means of abstraction such as naming, function definition, and objects
- Abstract away system details to be independent of hardware and operating system
Pharsing 解析
对于 scheme 表达式，首先把它分解成词元，然后再根据语法用 python 中自己定义好的 Pair 类进行组合
