Strange behavior in Lattice when superimposing

Hi,

I am trying to do a simple superimposing of two density plots and am
seeing strange behavior (I haven't seen this reported elsewhere).
This little snipit will make it apparent...

a = rnorm(10)
b = rnorm(100, mean=-2, sd=0.5)
densityplot( ~ a + b)

I'm not sure if this is explicitly documented anywhere (perhaps not), 
but the variables in the formula have to have the same length. What you 
should really do is 

densityplot(~ c(a, b), groups = rep(c('a', 'b'), c(10, 100))

Your usage should have triggered an error; I'll see if I can make that 
happen.

Deepayan