There are two forced numbers in Shortz 301: 1
in c (2,3) and 9 in c (2,6). After entering these
numbers into the puzzle, we mark up the puzzle.
The markup of a Sudoku puzzle can be done either
manually or by a computer program. The sensible
approach is to use a computer program for this
tedious task, which is prone to error when done
manually.3 The markup of Shortz 301, along with
the inclusion of the two forced numbers, is shown
in Figure 7.