dropUnusedLevels: Create Temporary Factor Subsetting Function
Description
Calling this function makes Hmisc have its pre-version 3.0 behavior in
which the R[.factor function was overridden by a customized
version that caused unused factorlevels to be dropped
when the factor variable was subscripted (subsetted).
dropUnusedLevels() creates a temporary version of [.factor
in the global environment, which will take precedence. To later make
this function keep unused levels on subsetting, issue
options(drop.unused.levels=FALSE) or just remove this temporary
function from the environment by issuing
remove('[.factor',pos='.GlobalEnv').