ab wrote:Suppose you pick the right choice first. Using this technique is only logical if you then go on to check all other possibilities, something I doubt many solvers would do

[ an earlier discussion of guessing — one of many! — can be found mixed into the Topic I don't even know how to post a message for many members! ] quoting from that Topic:

Pat wrote:em wrote:You can’t do just one chain and leave it at that - they require a proof by contradiction, don't they?MCC wrote:They do.

yes, when i find myself forced to use a proof-by-contradiction, i do always insist on developing both branches

yet reading the discussions on Deducing-From-Uniqueness-Of-Answer, you could conclude that the extra Given (i.e. we are given that the puzzle has only one answer) permits ignoring the 2nd branch whenever the 1st branch did happen to produce an answer!