 Usage:  dy=itediff(ytt,i)  
 
 Input:

  ytt                    k x n matrix, the transformed (and transposed) 
                         time series 
                         
  i                      integer 
                         
 Output:

  dy                     matrix, differenced time series 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
