 Usage:  {ur, x} = chfunc(usel, oldx, n, sel)  
 
 Input:

  usel                   vector commands 
                         
  oldx                   m x 1 array old data 
                         
  n                      scalar number of new datapoints 
                         
  sel                    k x 1 array 
                         
 Output:

  ur                     vector commands 
                         
  x                      m x 1 array new data 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
