 Usage:  y = tdiff(x {,d [, k]})  
 
 Input:

  x                      vector 
                         
  d (optional)           integer; the difference lag x(t)-x(t-d) 
                         default d=1 
                         
  k (optional)           integer; the number of differences 
                         
 Output:

  y                      vector; the differenced time series 
                         y(t)=(1-B^d)^k x(t) where BX(t)=X(t-1) 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
