Killer Sudoku

1. 1The sudoku rules hold: every row, column and 3 by 3 box holds the digits 1 to 9 exactly once. Few or no digits are given; the dotted cages tell you the rest.
2. 2The small number in a cage's corner is the total of its digits. A digit never repeats inside a cage.
3. 3Work out which digits can make a total. A 2-square cage of 3 can only be 1 and 2; a 2-square cage of 17 can only be 8 and 9. The Cage sums button lists the sets for the cage you select: it is free, it explains, it never fills a square.
4. 4Every row, column and box adds up to 45. When the cages inside a box cover all but one square, that square is 45 minus their totals: experienced players call it the rule of 45.
5. 5Check my work marks the digits that are wrong. A hint fills in one square: it costs a little XP, never time.
6. 6There is a new puzzle every day, the same for everyone: easy on Monday and Tuesday, medium on most days, hard on Saturday. Every puzzle has one solution and can be solved by logic alone.

Keyboard: arrows to move, 1 to 9 to write, N for notes, C for the cage sums, Backspace to erase, U to undo.