lots of work to get next thing;
    # possibly giving up for any of several reasons
    goto TOP   if  thing isn't interesting;
    if(  thing is invalid  ) {
        instruct;
        goto TOP;
    }
    do lots of stuff with thing;
    goto TOP   if  not last thing;
    if(  no valid things found  ) {
        complain;
        return;
    }
    finish up;
    return result;