It should make sure all open ends of tree are valid (contain no contradictions) and completed (all filled in) board states (of which there may be 0 (no solutions to puzzle), 1 (unique solution to puzlze), or more than 1 (multiple solutions to puzzle))
It should make sure all open ends of tree are valid (contain no contradictions) and completed (all filled in) board states (of which there may be 0 (no solutions to puzzle), 1 (unique solution to puzlze), or more than 1 (multiple solutions to puzzle))