Box 5 has four cells highlighted in blue at D5, D6, F5 and F6. The candidates 1, 2, 6 and 7 which appear in these cells do not appear
elsewhere in the box, and must therefore provide the solutions for the four highlighted cells. Any other candidates in these cells (the ones
highlighted in magenta) cannot be solutions, and may be removed.
Hidden quads can also appear in rows and columns.