The solving of a SUDOKU puzzle is guided by a red focus cell which can be moved about the puzzle by pointing and clicking with the mouse
or by means of the cursor keys.
Any character which is typed at the keyboard will be placed into the focus cell.
Under default conditions, the program displays in each unsolved cell, the set of candidate symbols which might legally be placed
into that cell. Whenever a solution symbol is placed into one of the cells, the list of candidate symbols in other cells is automatically updated.
If you are familiar with some of the more advanced SUDOKU solving techniques (X wing, Swordfish etc), you may see that certain of
the listed candidate symbols cannot possibly be a solution for the cell in which they are shown. Such candidates may be removed by pointing and clicking
with the mouse. When the last candidate symbol is removed from a cell, it becomes the solution symbol for that cell.