ATM Example
you: curr_balance=get_balance(acct_ID)
you: withdraw_amt=read_amount()
you: curr_balance=curr_balance-withdraw_amt
so: curr_balance=get_balance(acct_ID)
so: withdraw_amt=read-amount()
so: curr_balance=curr_balance-withdraw_amt
so: put_balance(acct_ID,curr_balance)
so: deliver_bucks(withdraw_amt)
you: put_balance(acct_ID,curr_balance)
you: deliver_bucks(withdraw_amt)