-- Chapter 30 example: a BankAccount whose methods guard the balance.
local Account = {}
Account.__index = Account
function Account.new(owner)
    return setmetatable({ owner = owner, balance = 0 }, Account)
end
function Account:deposit(amount)
    if amount > 0 then self.balance = self.balance + amount end
end
function Account:withdraw(amount)
    if amount > 0 and amount <= self.balance then
        self.balance = self.balance - amount
        return true
    end
    return false
end
function Account:getBalance() return self.balance end

local acc = Account.new("Keiko")
acc:deposit(100)
acc:withdraw(30)
print(acc:getBalance())   -- 70
-- TODO: add a :canAfford(amount) method that returns true when the
-- balance is at least amount, and use it before withdrawing.