class Email:
    """An email has the following instance attributes:

        msg (str): the contents of the message
        sender (Client): the client that sent the email
        recipient_name (str): the name of the recipient (another client)
    """
    def __init__(self, msg: str, sender, recipient_name: str):
        self.msg = msg
        self.sender = sender
        self.recipient_name = recipient_name

class Server:
    """Each Server has one instance attribute called clients that is a
    dictionary from client names to client objects.

    >>> s = Server()
    >>> # Dummy client class implementation for testing only
    >>> class Client:
    ...     def __init__(self, server, name):
    ...         self.inbox = []
    ...         self.server = server
    ...         self.name = name
    >>> a = Client(s, 'Alice')
    >>> b = Client(s, 'Bob')
    >>> s.register_client(a) 
    >>> s.register_client(b)
    >>> len(s.clients)  # we have registered 2 clients
    2
    >>> all([type(c) == str for c in s.clients.keys()])  # The keys in self.clients should be strings
    True
    >>> all([type(c) == Client for c in s.clients.values()])  # The values in self.clients should be Client instances
    True
    >>> new_a = Client(s, 'Alice')  # a new client with the same name as an existing client
    >>> s.register_client(new_a)
    >>> len(s.clients)  # the key of a dictionary must be unique
    2
    >>> s.clients['Alice'] is new_a  # the value for key 'Alice' should now be updated to the new client new_a
    True
    >>> e = Email("I love 61A", b, 'Alice')
    >>> s.send(e)
    >>> len(new_a.inbox)  # one email has been sent to new Alice
    1
    >>> type(new_a.inbox[0]) == Email  # a Client's inbox is a list of Email instances
    True
    """
    def __init__(self):
        self.clients = {}

    def send(self, email: Email):
        """Append the email to the inbox of the client it is addressed to.
            email is an instance of the Email class.
        """
        ____.inbox.append(email)

    def register_client(self, client):
        """Add a client to the clients mapping (which is a 
        dictionary from client names to client instances).
            client is an instance of the Client class.
        """
        ____[____] = ____

class Client:
    """A client has a server, a name (str), and an inbox (list).

    >>> s = Server()
    >>> a = Client(s, 'Alice')
    >>> b = Client(s, 'Bob')
    >>> a.compose('Hello, World!', 'Bob')
    >>> b.inbox[0].msg
    'Hello, World!'
    >>> a.compose('CS 61A Rocks!', 'Bob')
    >>> len(b.inbox)
    2
    >>> b.inbox[1].msg
    'CS 61A Rocks!'
    >>> b.inbox[1].sender.name
    'Alice'
    """
    def __init__(self, server: Server, name: str):
        self.inbox: list = []
        self.server = server
        self.name = name
        server.register_client(____)

    def compose(self, message: str, recipient_name: str):
        """Send an email with the given message to the recipient."""
        email = Email(message, ____, ____)
        self.server.send(email)

class VendingMachine:
    """A vending machine that vends some product for some price.

    >>> v = VendingMachine('candy', 10)
    >>> v.vend()
    'Nothing left to vend. Please restock.'
    >>> v.add_funds(15)
    'Nothing left to vend. Please restock. Here is your $15.'
    >>> v.restock(2)
    'Current candy stock: 2'
    >>> v.vend()
    'Please add $10 more funds.'
    >>> v.add_funds(7)
    'Current balance: $7'
    >>> v.vend()
    'Please add $3 more funds.'
    >>> v.add_funds(5)
    'Current balance: $12'
    >>> v.vend()
    'Here is your candy and $2 change.'
    >>> v.add_funds(10)
    'Current balance: $10'
    >>> v.vend()
    'Here is your candy.'
    >>> v.add_funds(15)
    'Nothing left to vend. Please restock. Here is your $15.'

    >>> w = VendingMachine('soda', 2)
    >>> w.restock(3)
    'Current soda stock: 3'
    >>> w.restock(3)
    'Current soda stock: 6'
    >>> w.add_funds(2)
    'Current balance: $2'
    >>> w.vend()
    'Here is your soda.'
    """
    def __init__(self, product: str, price: int):
        """Set the product and its price, as well as other instance attributes."""
        "*** YOUR CODE HERE ***"

    def restock(self, n: int) -> str:
        """Add n to the stock and return a message about the updated stock level.

        E.g., Current candy stock: 3
        """
        "*** YOUR CODE HERE ***"

    def add_funds(self, n: int) -> str:
        """If the machine is out of stock, return a message informing the user to restock
        (and return their n dollars).

        E.g., Nothing left to vend. Please restock. Here is your $4.

        Otherwise, add n to the balance and return a message about the updated balance.

        E.g., Current balance: $4
        """
        "*** YOUR CODE HERE ***"

    def vend(self) -> str:
        """Dispense the product if there is sufficient stock and funds and
        return a message. Update the stock and balance accordingly.

        E.g., Here is your candy and $2 change.

        If not, return a message suggesting how to correct the problem.

        E.g., Nothing left to vend. Please restock.
              Please add $3 more funds.
        """
        "*** YOUR CODE HERE ***"

    def time_to_retire(self, amount: float) -> int:
        """Return the number of years until balance would grow to amount."""
        assert self.balance > 0 and amount > 0 and self.interest > 0
        "*** YOUR CODE HERE ***"

class FreeChecking(Account):
    """A bank account that charges for withdrawals, but the first two are free!

    >>> ch = FreeChecking('Jack')
    >>> ch.balance = 20
    >>> ch.withdraw(100)  # First one's free. Still counts as a free withdrawal even though it was unsuccessful
    'Insufficient funds'
    >>> ch.withdraw(3)    # Second withdrawal is also free
    17
    >>> ch.balance
    17
    >>> ch.withdraw(3)    # Now there is a fee because free_withdrawals is only 2
    13
    >>> ch.withdraw(3)
    9
    >>> ch2 = FreeChecking('John')
    >>> ch2.balance = 10
    >>> ch2.withdraw(3) # No fee
    7
    >>> ch.withdraw(3)  # ch still charges a fee
    5
    >>> ch.withdraw(5)  # Not enough to cover fee + withdraw
    'Insufficient funds'
    """
    withdraw_fee: int = 1
    free_withdrawals: int = 2

    "*** YOUR CODE HERE ***"