[R] How to insert vertical space between lines of tables created with the R package 'tables'?

[Marius Hofert]

I have the data frame...

df <- cbind(expand.grid(d=as.factor(c(2,5)), n=c(100, 200),
beta=as.factor(c(0.2, 0.8)), group=LETTERS[1:2]), value=runif(16))

... which I would like to display in a table like ...

require(tables)
tabular(d * beta ~ group * mean * Heading() * value, data=df)

Now I would like to insert (vertical) space between every other line of the
table. I checked the manual and vignette of 'tables', but it only covers the
case when there is just one variable specified in the left-hand side of the
formula (so "to the left" of "~"). I tried to adapt the idea of using
RowFactor() ... 

tabular(RowFactor(1:4, "", spacing=2, space=0.5) + d * beta ~ group * mean *
Heading() * value, data=df)

... but obtain ... 

Error in `colnames<-`(`*tmp*`, value = "") : 
  length of 'dimnames' [2] not equal to array extent


Cheers,

Marius