Suppose that the striking teachers featured in this video agree to go back to work when the city of Chicago offers them a 20% pay increase over 5 years, or 4% per year. For the first three years, the city raises the teachers' salaries as agreed, but in the fourth year it increases teachers' salaries by only 1%. Teachers are shocked when they get their first paycheck of the year, because they were expecting the agreed-upon 4% increase. Which infraction has the city committed