time series - R, lag( ) has inconsistent behavior for xts and ts objects -

i take lag of xts variable, , lag() function returns lag. however, if use on ts variable, gives lead. bug, or working intended?

library('xts')  = as.xts(ts(c(5,3,7,2,4,8,3), start=c(1980,1), freq=4)) cbind(a, lag(a)) # provides lag 1 #         ..1 ..2 # 1980 q1   5  na # 1980 q2   3   5 # 1980 q3   7   3 # 1980 q4   2   7 # 1981 q1   4   2 # 1981 q2   8   4 # 1981 q3   3   8 b = ts(c(5,3,7,2,4,8,3), start=c(1980,1), freq=4) cbind(b, lag(b)) # provides lead 1 #          b lag(b) # 1979 q4 na      5 # 1980 q1  5      3 # 1980 q2  3      7 # 1980 q3  7      2 # 1980 q4  2      4 # 1981 q1  4      8 # 1981 q2  8      3 # 1981 q3  3     na 

as pointed out in documentation ?lag.xts, intended behavior.