A precondition is a condition that must be true before something is done
- I must have money before I can deposit it into an account.
A postcondition is something that is true after an action is done.
- My account will have greater value after I deposit my money
We want to describe any functions we write in terms of preconditions and postconditions.
We would also like to add comments to parameters telling us the way data is flowing

Example:

    void deposit_check(  /* in-out */ int& account_balence,
                         /* in */ int deposit_amount){

    // Preconditions:
    //     Both account_balence and deposit_ammount are initialized
    //     account_balence is set to the current balence
    //     deposit_amount holds an amount of money to deposit
    // Postconditions:
    //     account_balence is increased by deposit_amount

          account_balence += deposit_amount
    }