A guy on a log.
A log underneath a guy.
To be on a log you must be above and resting on the log, therefore the log is underneath the guy.
exists x1 x2.(a_topic(x1) & on(x1,x2) & n_log(x2) & n_guy(x1))
exists x1 x2.(a_topic(x1) & underneath(x1,x2) & n_guy(x2) & n_log(x1))