class Visitor:
    @contextlib.contextmanager
    def __call__(self, node):
        yield node

def visit(visitor, node):
    with visitor(node=node):
        for field in node.subnodes:
            subnode = getattr(node, field.name)
            with visitor(node=subnode):
                pass

def walk(root):
    nodes = _collections.deque([root])
    while nodes:
        node = nodes.popleft()
        nodes.extend(node.subnodes)
        yield node