demeter_delegation_2.rb

class Wallet
  attr_reader :credit_balance # no longer attr_accessor!
  def withdraw(amount)
     raise InsufficientFundsError if amount > @credit_balance
     @credit_balance -= amount
     amount
  end
end
class Moviegoer
  # behavior delegation
  def pay(amount)
    wallet.withdraw(amount)
  end
end
class MovieTheater
  def collect_money(moviegoer, amount)
    @collected_amount += moviegoer.pay(amount)
  end
end