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