code: def value( self, node, parent=None ):
          
          # this is the *weighted* size/contribution of the node 
          try:
              return node['contribution']
          except KeyError, err:
              contribution = int(node.get('totsize',0)/float( len(node.get('parents',())) or 1))
              node['contribution'] = contribution
              return contribution
docs: Return value used to compare size of this node