@classmethod
def class_key(cls):
    """Nice order of classes."""
    from ..sets.fancysets import Naturals0
    funcs = {'log': 11, 'sin': 20, 'cos': 21, 'tan': 22, 'cot': 23, 'sinh':
        30, 'cosh': 31, 'tanh': 32, 'coth': 33, 'conjugate': 40, 're': 41,
        'im': 42, 'arg': 43}
    name = cls.__name__
    try:
        i = funcs[name]
    except KeyError:
        i = 0 if isinstance(cls.nargs, Naturals0) else 10000
    return 4, i, name
