Freecell would be a perfect example of how to tune difficulty. Consider: some initial layouts can be solved without using all four free cells; others can't be

Freecell would be a perfect example of how to tune difficulty. Consider: some initial layouts can be solved without using all four free cells; others can't be solved without five or more cells. If a Freecell game implements a solver that can find the minimum number of cells needed to solve a given layout, then it could easily tune the difficulty - by only giving players layouts that require certain numbers of cells, or conversely by varying the number of free cells depending on the layout.