diffsn
Seasonal Differencing.
y=diff(x,s)
y=diff(x,s,k)
Inputs
x A vector or a matrix.
s An integer.
k A positive integer whose default value is assumed to be 1 .
Outputs
y A vector or matrix of numbers computed by differencing the input k times.


Description
For a vector input, diffsn(x,s) returns the array
    [ x[s+1]-x[1] , x[s+2]-x[2] , x[s+3]-x[3] ....].
If x is a matrix, the finite differnces are calculated column by column.

diffsn(x, s k) calculate the k'th difference by differencing the input array k times.