Previously, the set Comparator behaved the same way, regardless of the
value of columnFirst. This is incorrect; the set should be sorted such
that the final item in the set has the greatest cross-dimension.
There was also an off-by-one error in the result of
getHighestCrossDimension(). The highestCrossDimension data member is
exclusive, but the cell values in the occupiedCells set are inclusive.
We need to add 1 to the maximum cell cross-dimension in order to convert
it to an exclusive value.
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