'Check that ``maximum_retry_count`` is positive or None
:raises ValueError: If maximum_retry_count is invalid.'

def __invariant__(self):

    if (self.maximum_retry_count is not None):
        if (not isinstance(self.maximum_retry_count, int)):
            return (False, ('maximum_retry_count    must    be    an    integer    or    None,    got    %r' % (self.maximum_retry_count,)))
        if (self.maximum_retry_count < 1):
            return (False, ('maximum_retry_count    must    be    positive,    got    %r' % (self.maximum_retry_count,)))
    return (True, '')
