Tired of shoveling Sheryl Poulsen cleans up messes left by stray cats on her lawn in suburban Cottonwood Heights. Poulsen says the cats regularly invade her neighborhood - usually in the evening - and that she is tired of her lawn being used as a litter box. Animal experts say stray cats are multiplying at an alarming rate valleywide and that Poulsen's experiences are typical. (See story on B3.)