I am naive on the language and have not used it. At first I thought you could get around using `old` by assigning the new balance to a property `new_balance`,

I am naive on the language and have not used it. At first I thought you could get around using `old` by assigning the new balance to a property `new_balance`, but that won't work since this is an object's property so of course it must be set. I see the value and purpose now.