[R] List of "occurrence" matrices

[Lauri Nikkinen]

R users,

I don't know if I can make myself clear but I'll give it a try. I have
a data.frame like this

x <- "var1,var2,var3,var4
a,b,b,a
b,b,c,b
c,a,a,a
a,b,c,c
b,a,c,a
c,c,b,b
a,c,a,b
b,c,a,c
c,a,b,c"
DF <- read.table(textConnection(x),  header=T, sep=",")
DF

and I would like to sum all the combinations/occurences by a factor
(letter in this case) between variables and produce a list of
"occurrence" matrices. For example in this case the "occurrence"
matrix (first element of list) for factor "a" should look like this

>occulist
$a
	var1	var2	var3	var4
var1	x	0	1	1
var2	0	x	1	2
var3	1	1	x	1
var4	1	2	1	x

$b
etc.

because there is two rows where var2 and var4 has "a"

DF[DF$var2=="a" & DF$var4=="a",]

Can you give an advice how to achieve this kind of a list of matrices?

-Lauri