def value( self, node, parent=None ):
        """Return value used to compare size of this node"""
        # 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
Return value used to compare size of this node