In this lab, you will work with structs and contracts.

Download account.rkt and use-account.rkt from the course website.

Note that our account struct is immutable -- all operations return a changed structure without modifying the original.  Racket automatically creates some utility functions for your structs -- account-balance is a function that gets the balance field from the account, and account? is a boolean that verifies that you have an account.

In this lab, you must create contracts to:
*Verify that only accounts are passed as the first argument to balance, withdraw, and deposit.
*Verify that only positive numbers are passed as the second argument to withdraw and deposit.
*Verify that withdraw and deposit return an account.
*Verify that balance returns a number.
*Verify that the amount passed to withdraw does not exceed the available funds.