'Internal helper to ask whether a configuration is stronger than another.
A configuration is stronger when it contains every name/value pair in
kwargs.
Example: a configuration with:
(deadline=5, on_configuration=None, read_policy=EVENTUAL_CONSISTENCY)
is stronger than:
(deadline=5, on_configuration=None)
but not stronger than:
(deadline=5, on_configuration=None, read_policy=None)
or
(deadline=10, on_configuration=None, read_policy=None).
More formally:
- Any value is stronger than an unset value;
- Any value is stronger than itself.
Returns:
True if each of the self attributes is stronger than the
corresponding argument.'

def __is_stronger(self, **kwargs):

    for (key, value) in kwargs.iteritems():
        if ((key not in self._values) or (value != self._values[key])):
            return False
    return True
