标示符的命名规则
1. 字母或下划线开头
2. 以操作符开头，且只包含操作符（+ - * / # !等）
3. 用反引号`....`包括的任意字符串，即使是 Scala 关键字（39 个）也可以
• package, import, class, object, trait, extends, with, type, for
• private, protected, abstract, sealed, final, implicit, lazy, override
• try, catch, finally, throw
• if, else, match, case, do, while, for, return, yield
• def, val, var
• this, super
• new
• true, false, null