List seems to drop empty levels of factors when containing them

Greetings R community,

I am curious about the following behaviour: if I define a factor, and
then store a subset of it in a list, the stored version seems to drop
levels that were not included in the subset.  E.g. ..

mask <- c(T, F)
grp.1 <- factor(c(1,2))
list(grp.1)

[[1]]
[1] 1 2
Levels: 1 2

list(grp.1[mask])

[[1]]
[1] 1
Levels: 1

It is as though the list were redefining the factor and dropping
empty levels.  I would like to keep them (I am using the list for a
two-dimensional tapply). Is there any way to avoid this?

I'm using R 2.0.0 on FreeBSD 5.2.1.

Thanks,

Andrew

Andrew Robinson                      Ph: 208 885 7115
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University of Idaho                  E : andrewr at uidaho.edu
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