651cc9f6c7
Each constraint is a little sub-program that attempts to reduce the search space. They should only be run as required, i.e. when the candidates of the variables they use were updated, though this has not yet been implemented. Constraints are traits so that puzzles may implement their own specialised contraints as required. We have split the trait into an "on_assigned" call and an "on_updated" call for clarity. The provided methods simply return true, indicating no contradiction.