def demos():
    """Demos.

    >>> b = Account('Ada')
    >>> f = b.deposit
    >>> f(5)
    5
    >>> f(25)
    30
    >>> b.balance
    30
    >>> a = Account('Alan')
    >>> [a.deposit(n) for n in range(10)]
    [0, 1, 3, 6, 10, 15, 21, 28, 36, 45]
    >>> f = a.deposit
    >>> [f(n) for n in range(5)]
    [45, 46, 48, 51, 55]
    >>> a.balance
    55
    >>> d = {1: 10, 2: 5, 3: 15, 4: 8, 5: 4}
    >>> max(d.keys(), key=d.get)
    3
    """

class Town:
    """Waldo in town.

    >>> Town(1, 7).street[2]
    'Waldo'
    """
    def __init__(self, w, aldo):
        if aldo == 7:
            self.street = {self.f(w): 'Waldo'}

    def f(self, x):
        return x + 1

class Beach:
    """Waldo at the beach.
    
    >>> Beach().walk(0).wave(0)
    'Waldo'
    """
    def __init__(self):
        sand = ['Wal', 'do']
        self.dig = sand.pop

    def walk(self, x):
        self.wave = lambda y: self.dig(x) + self.dig(y)
        return self

class Account:
    """An account has a balance and a holder.
    All accounts share a common interest rate.

    >>> a = Account('John')
    >>> a.holder
    'John'
    >>> a.deposit(100)
    100
    >>> a.withdraw(90)
    10
    >>> a.withdraw(90)
    'Insufficient funds'
    >>> a.balance
    10
    >>> a.interest
    0.02
    >>> Account.interest
    0.02
    """
    interest = 0.02

    def __init__(self, account_holder):
        self.holder = account_holder
        self.balance = 0

    def deposit(self, amount):
        """Add amount to balance."""
        self.balance = self.balance + amount
        return self.balance

    def withdraw(self, amount):
        """Subtract amount from balance if funds are available."""
        if amount > self.balance:
            return 'Insufficient funds'
        self.balance = self.balance - amount
        return self.balance

from dataclasses import dataclass

@dataclass
class Email:
    msg: str
    sender: str
    recipient: str

class Server:
    """The server's clients can send messages to each other.

    >>> s = Server()
    >>> a = Client(s, 'Alice')
    >>> b = Client(s, 'Bob')
    >>> a.compose('Hello!', 'Bob')
    >>> b.inbox[0]
    Email(msg='Hello!', sender='Alice', recipient='Bob')
    """
    def __init__(self):
        self.clients: dict[str, Client] = {}

    def send(self, email: Email) -> None:
        "Append the email to the inbox of the recipeint's client."
        self.clients[email.recipient].inbox.append(email)

class Client:
    """A client adds itself to the server when constructed."""
    def __init__(self, server: Server, name: str):
        self.server = server
        self.name = name
        self.inbox: list[Email] = []
        server.clients[name] = self

    def compose(self, message: str, recipient: str) -> None:
        "Send an email with the given message to the recipient."
        self.server.send(Email(message, self.name, recipient))