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Sodokus are simple number puzzles that often appear in various sources of print.
The puzzle you are given is a 9 x 9 grid of numbers and blanks, that might look
something like this:

	+-------+-------+-------+
	| _ 6 _ | 1 _ 4 | _ 5 _ |
	| _ _ 8 | 3 _ 5 | 6 _ _ |
	| 2 _ _ | _ _ _ | _ _ 1 |
	+-------+-------+-------+
	| 8 _ _ | 4 _ 7 | _ _ 6 |
	| _ _ 6 | _ _ _ | 3 _ _ |
	| 7 _ _ | 9 _ 1 | _ _ 4 |
	+-------+-------+-------+
	| 5 _ _ | _ _ _ | _ _ 2 |
	| _ _ 7 | 2 _ 6 | 9 _ _ |
	| _ 4 _ | 5 _ 8 | _ 7 _ |
	+-------+-------+-------+

The task is to fill in the remaining digits (1 through 9 only) such that each
row, column, and 3 x 3 box contains exactly one of each digit.  Here's the
solution for the above puzzle:

	+-------+-------+-------+
	| 9 6 3 | 1 7 4 | 2 5 8 |
	| 1 7 8 | 3 2 5 | 6 4 9 |
	| 2 5 4 | 6 8 9 | 7 3 1 |
	+-------+-------+-------+
	| 8 2 1 | 4 3 7 | 5 9 6 |
	| 4 9 6 | 8 5 2 | 3 1 7 |
	| 7 3 5 | 9 6 1 | 8 2 4 |
	+-------+-------+-------+
	| 5 8 9 | 7 1 3 | 4 6 2 |
	| 3 1 7 | 2 4 6 | 9 8 5 |
	| 6 4 2 | 5 9 8 | 1 7 3 |
	+-------+-------+-------+

This week's Ruby Quiz is to write a solver that takes the puzzle on STDIN and
prints the solution to STDOUT.