Input is usually the comparison of two numbers.
At same time the computer shows the number one for true and 0 for false. In a account you enter credits and debits line by line. After each entry you want to know the current amount on this account. You can use the first project to work out, that the sum of debit and credit are grater than zero or that credit or debit are grater than zero, if there is a entry for credit or debit or both. Then you show that one can trigger to pick one of two values by the outcome of a comparison and make up the entire model.