Ability to remove a coin

[alxwrd]

Coins can be created by subclassing Coin, but once created they can't be removed.

See: https://stackoverflow.com/q/52428679/7220776

[alxwrd]

I like the comment from @PatrickHugh:

Instead of manipulating __subclasses__, I would recommend keeping your own list instead, possible through the __init_subclass__ hook

class Coin:
    subcoins = []

    def __init_subclass__(cls):
        Coin.subcoins.append(cls)

    @classmethod
    def remove_subcoin(cls, coin):
        Coin.subcoins.remove(cls)

Then instead of Coin.__subclasses__(), use Coin.subcoins.

[alxwrd]

Coin already has a method for getting sub coins, added in 8b7aa0e: Coin.sub_coins().

Might be best to mark the list tracking the coins private.

__init_subclass__ should also ensure the coin isn't already in _subcoins. So maybe this should be a set?

class Coin:
    _subcoins = set()

    def __init_subclass__(cls):
        Coin._subcoins.add(cls)

    @classmethod
    def remove_subcoin(cls, coin):
        Coin._subcoins.remove(cls)