Comment by bluetomcat

Interesting usage of "extern" and "auto". Quite different from contemporary C:

    tree() {
        extern symbol, block, csym[], ctyp, isn,
        peeksym, opdope[], build, error, cp[], cmst[],
        space, ospace, cval, ossiz, exit, errflush, cmsiz;

        auto op[], opst[20], pp[], prst[20], andflg, o, p, ps, os;
        ...

Looks like "extern" is used to bring global symbols into function scope. Everything looks to be "int" by default. Some array declarations are specifying a size, others are not. Are the "sizeless" arrays meant to be used as pointers only?