maho: Although the R17 sequence works, in a whole-board game doesn't it result in some slight bad aji for white due to reduced liberties for its group? Plus if black doesn't defend at R18 there is a chance it could get a forcing move on the outside, threatening to connect underneath. So while it is a valid sequence it seems slightly worse than the direct kill at T18
posetcay: It is fine as long as both kill, this is a beginner's collection.
It is fine as long as both kill, this is a beginner's collection.