Hi,
it occurred to me that we have the diff/cum pair of functions, but for
seasonal periodicities we only have sdiff, not "scum" (which maybe
wouldn't be an ideal name anyway ;-)
Maybe the cum() function could be extended with an optional argument
that would allow to invoke a seasonal-step-style cumulation? For
example, for quarterly data the result would be out(t) = in(t) +
in(t-4), such that sdiff(out) would give you back 'in'.
thanks
sven