[R] modify the levels of a variable

[clee]

If anyone can help with this, I would greatly appreciate it.

When I subset a dataframe, and then create a table of on of the column
variables, the levels of that variable are retained from the parent
dataframe.  Anyone know how to 


for example, say I have data like so:

data:
X     Y
a     2
b     2
a     3
b     3
a     5

if I do....

data2=subset(data,X=="a")

I get the new data frame:

data2:
X     Y
a     2
a     3
a     5

but if I do this...

table(data2$X)

I would get this:

X     Y
3     0

additionally when I write...

levels(data2$X)

I get...
Factors w/ 2 levels: "a", "b"

I want it to be...
Factors w/ 1 level: "a"

Normally I would just move on and won't even bother but I'm working with a
variable that has 204 levels and only 84 of those are meaningful to me.  How
can I reset the levels of a column variable?  help please!  Thanks!
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