 Usage:  subcs = prune (cs, alfa)  
 
 Input:

  cs                     list of vectors: data structure which represents a binary 
                         tree and is produced by cartsplit procedure, contains vectors 
                         cs.val, cs.vec, cs.mean, cs.ssr, cs.nelem. 
                         See cartsplit for the description of cs. 
                         
  alfa                   real number >= 0: the tree subcs is the subtree 
                         of cs which minimizes the error-complexity criterion 
                         with the complexity parameter alfa. 
                         
 Output:

  subcs                  similar kind of object than cs, see cartsplit for the 
                         description of subcs. The tree subcs is the subtree of cs 
                         which minimizes the error-complexity criterion when the 
                         complexity parameter is alfa. 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
