code: def add_node(self,label):
        '''Return a node with label. Create node if label is new'''
        try:
            n = self._nodes[label]
        except KeyError:
            n = Node()
            n['label'] = label
            self._nodes[label]=n
        return f add_node(self,label):
        '''Return a node with label. Create node if label is new'''
        try:
            n = self._nodes[label]
        except KeyError:
            n = Node()
            n['label'] = label
            self._nodes[label]=n
        return n
docs: Return a node with label. Create node if label is new