Sudoku solving technique: focus on a number

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When a house has still more than one empty cells you can still find certainties by combining the logic of multiple houses. Take look at the below example that focuses on the number 4:

8 2 3
9 4 2 7 3
8 2 5 9
9 7 3 8 1
3 7
6 4 7 3

In the 3rd row the only possible position to place the 4 is the fourth. Position 1,2 and 3 are blocked by the 4 that’s already in the first block. Position 6 is blocked by the 4 that’s already in the 6th column.