Complete the following partially filled 7x7 Latin square. Each row and column must contain the numbers 1-7 exactly once. Cells marked 0 are empty and must be filled.

Row 1: [0, 0, 2, 0, 0, 7, 0]
Row 2: [6, 3, 4, 0, 0, 5, 0]
Row 3: [3, 0, 0, 0, 2, 1, 0]
Row 4: [0, 0, 0, 0, 7, 3, 0]
Row 5: [0, 0, 0, 4, 3, 6, 0]
Row 6: [5, 1, 0, 0, 0, 0, 0]
Row 7: [7, 0, 3, 0, 4, 0, 0]

Implement a constraint-propagation solver with backtracking. Emit the completed square and verify that every row and column is a permutation of {1,...,7}.
