[R] how to fix a factor

[Giampiero Salvi]

Hi all,
I created a data frame with three factors, plus the response that
looks like this:

x1	x2	x3	y
a	1	1	0.3
a	2	1	0.1
b	1	1	0.4
c	4	3	0.1
...

I would like to analise the effect of two of them, keeping the third fixed
(I already know the effect of the last). The reason why I don't create several
data frames for each value of the thirs factor is simply convenience (I'd like
to be able to decide which factor I want to rule out)

for example I'd like to write something like

boxplot(y ~ x1 + x2, x3 == 1)

which of course doesn't work, otherwise I wouldn't write :-)

Is this possible and how?

Thank you
Giampiero