zoo arithmetics

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)