Reform a factor so that only necessary levels are kept.
a factor or a vector to be converted to a factor.
In general, the levels of a factor,
levels(x), may include values that never actually occur.
This function drops any levels of that do not occur.
x is not a factor, then the function returns
x is a factor, then the function returns the same value as
x[,drop=TRUE] but somewhat more efficiently.
A factor with the same values as
x but with a possibly reduced set of levels.
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