[R] how to include 'NA's in xtabs?

[Kosenkov Kirill]

Hello!

I have a dataset with NA's in some variables (factors), for example:
$ P67 : Factor w/ 2 levels "-","+": NA 2 1 NA NA 2 1 1 2 NA ...

I need to use 'xtabs' like
xtabs(~x$P67)
It works well and produces something like this:
x$P67
    -    +
  779 1318
but i want to compute NA's too, like this:
x$P67
    -    +   NA
  779 1318  137

I am trying xtabs(~x$P67, exclude=NULL) but xtabs does not compute 
'NA's in this case too.

I do not want to transform my data (do not want to do any 
substitution on NA's).

How i can say 'xtabs' to compute NA-values?

and second question: how to use argument 'na.action' in 'xtabs'?
'xtabs' help page does not explain this.

Thanks!