Sifco Inc., a tire manufacturing company, stipulates that Rambox Corp. should buy its entire line of Roadgrip tires. Sifco has significant market power in a particular variety of Roadgrip tires for which no substitutes are available in the market. It uses this power to its advantage and does not agree to let Rambox buy less than the entire line of Roadgrip tires. The sales contract between Sifco and Rambox is an example of a ________.