 Usage:  {xr,yr} = discrete(x{,y})  
 
 Input:

  x                      n x p matrix, the data matrix to reduce, in 
                         regression usually the design matrix. The 
                         matrix may be numeric or string, in the latter 
                         case no y is possible. 
                         
  y                      optional, n x q matrix, in regression 
                         usually the observations of the dependent 
                         variable. Not possible for string matrix x. 
                         
 Output:

  xr                     m x p matrix, reduced data matrix (sorted). 
                         
  yr                     m x 1 vector or m x (q+1) matrix, contains in the first 
                         column the number of replications. If y was given, sums 
                         of y-rows with same x-row are contained in the other 
                         q columns of r. 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
