senseia_a_ron: If I play at N5 that violates the ko rule, right?
caranthir: No, the ko rule says that you can't play a move that reproduces the immediately previous position (common), or any previous position (superko). In these problems we always assume the common ko rule, which in real games almost always works (except in case of a very rare, 1/10000-ish triple ko or other longer cycles, in which case the game is deemed void). Superko, on the other hand, works 100 % in preventing all cycles. Of a historical note, capturing at N5 would indeed violate a ko rule that was used in Tibetan go, which prohibits playing a stone at an intersection from which your stone was just removed.
No, the ko rule says that you can't play a move that reproduces the immediately previous position (common), or any previous position (superko). In these problems we always assume the common ko rule, which in real games almost always works (except in case of a very rare, 1/10000-ish triple ko or other longer cycles, in which case the game is deemed void). Superko, on the other hand, works 100 % in preventing all cycles. Of a historical note, capturing at N5 would indeed violate a ko rule that was used in Tibetan go, which prohibits playing a stone at an intersection from which your stone was just removed.