 Usage:  nar = haznar(data)  
 
 Input:

  data                   n x (p+4) matrix, the first column is the sorted 
                         survival time, followed by the sorted censoring 
                         indicator, labels l, a column containing the 
                         number of ties, and lastly, the sorted covariate 
                         matrix z. This data is produced by hazdat.xpl. 
                         
 Output:

  nar                    n x 1 vector, the ith entry is the size of the 
                         risk set at time t_i. 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
