[R] subsetting a dataframe

[yjmha69]

Hi there, 
> a<-data.frame(c(1,2,2,5,9,9),c("A","B","C","D","E","F")) 
> names(a)<-c("x1","x2") 
> max(table(a$x1)) 
[1] 2 
> 
The above shows the max count for x1 is 2, which is correct. But we can't tell 
there are 2 groups that meet this criteria: 2,2 and 9,9. 
I then want to extract the records that has the hightest count 
> a[max(table(a$x1)),] 
  x1 x2 
2  2  B 
This is not working, since it is equvalent to a[2,] 
What I want is 
   x1 x2 
2 2   B 
3 2   C 
5 9   E 
5 9   F 

I think this should be very easy, but I'm a beginner :-) 

Thanks 

YJM