Have a look at the highlighted square. That 4x4 region already contains 1, 2, 4, 5, 7, 9, D, E and F. The
column already contains 2, 4, 5, 7, 8, 9, B, C, D and F. The row already contains
2, 3, 4, 6, 8, 9, A, B, C, D, E, F. Combining all of those together leaves us with just the letter 'G'
that hasn't already been taken.
The highlighted cell must be a G!
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