 Usage:  y = qff (x, d1, d2)   
 
 Input:

  x                      array 
                         
  d1                     array or scalar 
                         
  d2                     array or scalar 
                         
                         
 Output:

  y                      array 
                         
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
