In version 1.5.7, <<= is made a synonym of << and in the trunk, it has become the other way around with a deprecation warning for <<. But the problem in that in both cases, __ilshift__ returns None. That means that with, at least with Python 2.x,
expr1 <<= expr2
results in expr1 being None. It can therefore not be reused in any subsequent rules.
Am I missing something, or is it a serious bug?