Julio and Juan became good friends while working for the Senor Tacos restaurant chain. The chain used a(n) method of operation, because its owner licensed others to use its trade name in making and selling tacos. Julio and Juan worked under the direction of Carmen, the , who had the license to operate the business, given to her by Senor Tacos, the . Every evening after work, Julio and Juan spent their time together inventing a greaseless taco fryer. Because this relationship involved two people combining their labor and property for a single business undertaking, it was known as a(n) , which is also called