 Usage:  res = ssr(cs, node)  
 
 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. 
                         
  node                   integer >= 1: gives the index of the root node of 
                         the subtree whose mean of squared residuals 
                         will be calculated. 
                         If node=1, the mean of squared residuals in the hole tree is 
                         calculated, if node=2, the mean of squared residuals in the 
                         left subtree is calculated, if node=3, the mean of squared 
                         residuals in the left subtree of the left subtree 
                         (if it exists) is calculated. See cartsplit for 
                         the explanation how the tree is represented as a 
                         vector. 
                         
 Output:

  res                    real number >= 0: sum of squared residuals in the specified 
                         subtree. 
                         
--------------------------------------------------------------
(C) MD*TECH Method and Data Technologies, 17.8.2000
