I noticed, using the visualizations of rating-sudoku project
(https://github.com/eric-brechemier/rating-sudoku) that advanced
strategies were used on sudoku puzzles that could be solved without
them: the current implementation will use a harder strategy to
identify a cell as soon as possible, which could be found with
an easier strategy, at the cost of extra steps.

In order to give an accurate representation of the difficulty
of sudoku puzzles, the implementation should delay the use of
harder strategies to a later step, when no progress can be made
with any easier strategy.

This behavior could be customized using an option parameter:
* shortest solution (using more complex strategies if needed)
* easiest path (using more steps if needed)
if the current behavior is to be preserved at all.

1.
I noticed, using the visualizations of rating-sudoku project
(https://github.com/eric-brechemier/rating-sudoku) that advanced
strategies were used on sudoku puzzles that could be solved without
them: the current implementation will use a harder strategy to
identify a cell as soon as possible, which could be found with
an easier strategy, at the cost of extra steps.

In order to give an accurate representation of the difficulty
of sudoku puzzles, the implementation should delay the use of
harder strategies to a later step, when no progress can be made
with any easier strategy.

This behavior could be customized using an option parameter:
* shortest solution (using more complex strategies if needed)
* easiest path (using more steps if needed)
if the current behavior is to be preserved at all.

2.
The visualization of Sudoku #69 from Puzzler (2006) in rating-sudoku
shows a high number of cells unidentified with any of the three current
strategies. This may be due to a glitch in current implementation,
or the sign that a new strategy needs to be added.

Add another insight gained from the rating-sudoku visualization
Add another insight gained from the rating-sudoku visualization
