simple for a medium

ez when you see the loners -- usually one of my weak points

TallMike - If all of the pairs were in one block, then you would have ab, ab, ab, and abc all in the same block, which by itself is an impossibility. My remembrance is that as soon as I backtracked and chose a different path from where I had used the UR, the puzzle was solved, so there was not an error previously. Of course, this was a while ago, and my memory could be in error. If you mean that the four corners of the UR were actually in four blocks, then you would be correct, but I would be surprised if that had fooled several of us experienced puzzlers.

Janet ... if the four corners of the rectangle are in two blocks then one of the four must have a number different from the other three or you will have the situation where there are two solutions, i.e. you cant tell if the solution goes 1-3 3-1 or 3-1 1-3. This is contrary to the standard rule of puzzle design which states that there can be only one solution.

However, if the four corners of the rectangle are in four different boxes, it is possible in fact, necessary that some other combination of the other cells will force only one of the numbers to be eligible the particular corner.

.... whatever happens to typing and spelling and grammar when you get into a computer chat ... sorry about the missing commas and parentheses ...

Slow, but I cant say anything was very tricky.