It's still cheating. Getting an advantage over the rest of the competition because of advanced notice of another player's move is clearly cheating. Proving it is quite another matter.
I suggest an independent panel. If someone thinks there has been cheating then the panel will adjudicate and eliminate the offending player(s). Proof isn't needed, just a fair hearing. :)
(piilota) Jos seuraat säännöllisesti vain joitakin keskustelualueita, voit lisätä ne suosikkikeskusteluihin klikkaamalla halutun palstan yläreunassa olevaa "lisää suosikkeihin" -linkkiä. (pauloaguia) (näytä kaikki vinkit)