A class for keeping double counts of {@link List}s of a prespecified length. A depth
n GeneralizedCounter can be thought of as a conditionalized count over
n classes of objects, in a prespecified order. Also offers a read-only view as a Counter.
This class is serializable but no guarantees are made about compatibility version to version.
This is the worst class. Use TwoDimensionalCounter. If you need a third, write ThreeDimensionalCounter, but don't use this.
@author Roger Levy