30 如何切换不同文类？想要切换ctexbook和book

%%% autoclass.cls
\ProvidesClass{autoclass}

\DeclareKeys{% = \DeclareKeys[autoclass]
  lang .choice: ,
  lang/en .code:n = \def\autoclass@class{book} ,
  lang/cn .code:n = \def\autoclass@class{ctexbook} ,
  lang .initial:n = en ,
  unknown .code:n = \autoclass@unknown ,
}

\ExplSyntaxOn
\clist_new:N \autoclass@unknownoptions
\cs_new_protected:Npn \autoclass@unknown
  {
    \bool_if:NTF \l__keys_no_value_bool
      { % eg. 12pt, twoside
        \clist_put_right:Nx \autoclass@unknownoptions
          { \l_keys_key_str }
      }
      { % eg. zihao=4, fontset=windows
        \clist_put_right:Nx \autoclass@unknownoptions
          { \l_keys_key_str = { \exp_not:o \l_keys_value_tl } }
      }
  }
\ExplSyntaxOff

\ProcessKeyOptions % = \ProcessKeyOptions[autoclass]
\PassOptionsToClass\autoclass@unknownoptions\autoclass@class
%% 如果不需要过滤上面定义的 lang 等键，可以直接使用
%% \PassOptionsToClass\@raw@classoptionslist\autoclass@class

\LoadClass{\autoclass@class}

%%% main.tex
\documentclass[lang=cn,fontset=fandol,12pt]{autoclass}
\begin{document}

Hello.
\ifdefined\CTeX \CTeX 中文 \fi

\end{document}