'Fails if the given ``string`` is not in lowercase.
For example, ``\'string\'`` and ``\'with specials!\'`` would pass, and
``\'String\'``, ``\'\'`` and ``\' \'`` would fail.
The default error message can be overridden with the optional
``msg`` argument.
See also `Should Be Uppercase` and `Should Be Titlecase`.'

def should_be_lowercase(self, string, msg=None):

    if (not string.islower()):
        self._fail(msg, "'%s'    is    not    lowercase.", string)
