This is the simplest strategy available for the solving of SUDOKU puzzles. So simple, it is really only included
here for the sake of completeness.
The cell highlighted in blue has the single number 7 "pencil marked" in. All of the other possible digits have already
been included in either the row, the column, or the 3x3 sub-square of which the highlighted cell is a member. It therefore
follows that 7 must be the solution value for this cell.