df <- data.frame(var1=c('a', 'b', 'c'), var2=c('d', 'e', 'f'), freq=1:3)

What is the simplest way to expand the first two columns of the data.frame above, so that each row appears the number of times specified in the column 'freq'?

In other words, go from this:

>df
  var1 var2 freq
1    a    d    1
2    b    e    2
3    c    f    3

To this:

>df.expanded
  var1 var2
1    a    d
2    b    e
3    b    e
4    c    f
5    c    f
6    c    f