[R] converting lists got by tapply to dataframes

[Wolfram Fischer]

I have two lists:

    xa <- list( X=c(1,2,3), Y=c(4,5,6), Z=c(7,8,9) )

    xb <- with( barley, tapply( X=seq(1:nrow(barley)), INDEX=site
        , FUN=function(z)yield[z]))

I can convert xa to a dataframe easily with:
    as.data.frame(xa)

But if i try the same with xb I get:
    as.data.frame(xb)
    Error in as.data.frame.default(xb) :
    can't coerce array into a data.frame

What helps?

(NB: I know the formula for xb is stupid, but it generates the same
type of list as the list I get from my real problem.)

Wolfram