Usage is a replacement usage function for the flags package.

实现的接口.

继承的类型，例如 `type jsonCodec Coder`，其中Coder就是typeName.

需要注册的对象，会将所有实现了 `Codec` 的对象注册到该实例内.

File holds a single parsed file and associated data.

Generator holds the state of the analysis.

Value represents a declared constant.