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[sdq_pete]

The pattern of today's puzzle has appeared a number of times with different starting positions and different grades, from easy to very hard - today's happens to be Medium.
This particular pattern lends itself to what I think of as Petes Corollary to the Left-over Law. The LOL is invariably a powerful technique in virtually every squiggly. Simply stated, if you can draw a line through the matrix between any two rows or columns such that some traversed boxes 'protrude' on one or other side of the line and an equal number of such protruded cells appears on each side, then the cells on each side form a subset, and the content of each of these two subsets is the same. I would prefer to call this the Subset Rule.

What I call "Pete's Corollary" (or provisionally perhaps, "Pete's Postulate") is this. If you draw such a line and the number of protruded cells on one side is 9, then these nine cells form a complete set of the digits 1 - 9. At first sight this may not appear interesting but in fact it is probably as useful as the initial LOL. By inspecting what digits already occur in the set, occurrences in other parts of the set can be eliminated.
The pattern of today's puzzle is a good example. Horizontal lines cut off the protruding arms of the central boxes, each arm containing 3 cells.

Just thought I'd get this off my chest!

Peter