Sorry, to elaborate, when actually logging, newer binds trump older binds:

    >>> from __future__ import print_function
    >>> log = BoundLog(lambda *args, **kwargs: print(args, kwargs), lambda: None)
    >>> log2 = log.bind(a=2, b=20).bind(a=10)
    >>> log2.msg('hi')
    ('hi',) {'a': 10, 'b': 20}