Dear list members, what is the reason that one obviously can't do arithmetic operations on zoo members with different index positions? > require(zoo) > z <- zoo(c(1,1,1),order.by=c(1,2,3)) > z 1 2 3 1 1 1 > z[1] + z[1] 1 2 > z[1:2] + z[1:2] 1 2 2 2 > z[1] + z[2] Data: numeric(0) Index: numeric(0)