[R] adding tables

[Ross Boylan]

Can anyone recommend a good way to add tables?
Ideally I would like
t1 <- table(x1)
t2 <- table(x2)
t1+t2

It t1 and t2 have the same levels this works fine, but I need something
that will work even if they differ, e.g.,
 > t1

 1 2 4 5
 2 1 1 1
 > t2 <- table(c(10, 11, 12, 13))
 > t1+t2  # apparently does simple vector addition

 1 2 4 5
 3 2 2 2
whereas I want
1 2 4 5 10 11 12 13
2 1 1 1  1  1  1  1