String-based arti cial chemistry that allows maintenance of di erent types of self-replicators

A cover assembly for an upwardly opening receptacle including opposite ends and longitudinally extending opposite side walls. The cover assembly includes an elongated flexible cover panel having one end portion anchored to one end of the receptacle and a pair of pulleys are journalled from the opposite end portions of the upper marginal edge portion of each side wall and have an endless flexible member trained thereabout. The other end portion of the flexible cover includes structure for attaching the opposite side marginal edge portions of the cover to the upper reaches of the corresponding endless tension members. Orbital movement of the endless flexible tension members about the pulleys causes longitudinal shifting of the upper reaches of the tension members and in this manner the free end portion of the flexible cover panel may be moved toward and away from the far end of the load receptacle.