为什么解析图6似乎在networkx中丢失?

NETWorkX docs 假设有一个函数用于解析调用的graph6字符串 parse_graph6 ( networkx.readwrite.graph6.parse_graph6 但它似乎不存在。这个功能消失了,还是我做错了什么?

jack@hitch:~$ pip show networkx | grep Version
Version: 2.1
jack@hitch:~$ python
Python 2.7.15rc1 (default, Nov 12 2018, 14:31:15) 
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import networkx
>>> dir(networkx.readwrite.graph6)
['NetworkXError', '__all__', '__builtins__', '__doc__', '__file__', '__name__', '__package__', '_generate_graph6_bytes', 'data_to_n', 'from_graph6_bytes', 'islice', 'n_to_data', 'not_implemented_for', 'nx', 'open_file', 'read_graph6', 'sys', 'to_graph6_bytes', 'write_graph6', 'write_graph6_file']

>>> networkx.readwrite.graph6.parse_graph6
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'parse_graph6'
>>>