Dominosa: add a Hard difficulty which can do set analysis.

Dominosa: add a Hard difficulty which can do set analysis.

This is another thing I've been doing with my own brain for ages as a
more interesting alternative to scouring the grid for the simpler
deduction that must be there somewhere - but now the solver can
understand it too, so it can generate puzzles that _need_ it (or at
least need something beyond the simpler strategies it understands).