1393
1369
data during reads, and allows a 'write_group' to be obtained. Write
1394
1370
groups must be used for actual data insertion.
1402
1372
:param token: if this is already locked, then lock_write will fail
1403
1373
unless the token matches the existing lock.
1404
1374
:returns: a token if this instance supports tokens, otherwise None.