def main():
    sys_bwrite('/etc/passwd', ('plain', 'secret...'))
    sys_bwrite('file', ('plain', 'data...'))

    pid = sys_fork()
    sys_sched()
    if pid == 0:
        # attacker: symlink file -> /etc/passwd
        sys_bwrite('file', ('symlink', '/etc/passwd'))
    else:
        # sendmail (root): write to plain file
        filetype, contents = sys_bread('file')  # for check
        if filetype == 'plain':
            sys_sched()  # TOCTTOU interval

            filetype, contents = sys_bread('file')  # for use
            match filetype:
                case 'symlink': filename = contents
                case 'plain': filename = 'file'
            sys_bwrite(filename, 'mail')
            sys_write(f'{filename} written')
        else:
          sys_write('rejected')