SteveG48 wrote :
Not quite. I don't see that there are 2 solutions if r9c5 <> 9. I see that if there is at least one solution with r9c5 <> 9, then there must be at least 2 solutions. A subtle distinction, but crucial. As you've shown, there are no solutions if r9c5 <> 9. This is what we expect, since we've assumed from the beginning that it's a proper puzzle with a unique solution. That's the basis of Phil's elimination, and all eliminations involving uniqueness arguments.
Spot on, and very refreshing to see that someone else understands uniqueness principles properly. Obviously your agreement with Phil's statement without qualification was just a linguistic quirk, to save on words.
Hopefully Phil's original statement that there would be 2 solutions if r9c5 <> 9 was a similar linguistic quirk.
Leren
