如果想要能够改变它的值，必须使用 mut 关键字使其可变。

fix_incorrect_order 函数在 back_of_house 模块中，所以我们可以使用 super 进入 back_of_house 父模块，也就是本例中的 crate 根。

transforms the resulting HTTP responses into Observables.

options object that is only supplied at run-time

constructor(private catsService: CatsService) {}

the providers that will be instantiated by the Nest injector

cat.interface.ts

include the HTTP_OPTIONS custom token. P

'CONNECTION'

the registration process.

The @Injectable() decorator attaches metadata, which declares that CatsService is a class that can be managed by the Nest IoC container.