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  Minesweeper Consistency Solver
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  The minesweeper consistency problem is stated as follows:
  Given a ROWSxCOLS minesweeper grid of some exposed tiles numbering the mines
surrounding them, does there exist a valid placement of n mines?

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  TODO
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1. Order unknowns: Order unknowns by how constrained they are, and assign
   values to the most constrained ones first. This allows for more pruning at
   the top of the search tree.

2. Derive values: Once an UNKNOWN is assigned to ON or OFF, other unknowns may
   now only have one legal value. Force the assignment of that value.

3. Likely value: In case both ON and OFF are valid, attempt to judge what is
   the most likely value, and assign that one first.