[Rd] stack problem

[Gabor Grothendieck]

stack() seems to drop empty levels.  Perhaps there could be a
drop=FALSE argument if one wanted all the original levels.  In the
example below, we may wish to retain level "b" in s$ind even though
component LL$b has length 0.

> LL <- list(a = 1:3, b = list())
> s <- stack(LL)
> str(s)
'data.frame':   3 obs. of  2 variables:
 $ values: int  1 2 3
 $ ind   : Factor w/ 1 level "a": 1 1 1


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