'Add a node to the scheme. An error is raised if the node is
already in the scheme.
Parameters
node : :class:`.SchemeNode`
Node instance to add to the scheme.'

def add_node(self, node):

    check_arg((node not in self.__nodes), 'Node    already    in    scheme.')
    check_type(node, SchemeNode)
    self.__nodes.append(node)
    log.info(('Added    node    %r    to    scheme    %r.' % (node.title, self.title)))
    self.node_added.emit(node)
