Some sections of the city of Manitou Springs have hills of varying degrees of slope. To protect against water drainage and erosion, the city adopted a hillside zoning ordinance that required homes on hillsides to be surrounded by more open land than in the balance of the city. Sellon owned land on a hillside and claimed that the hillside ordinance was unconstitutional because it did not treat all homeowners equally. Was the ordinance valid? [Sellon v. City of Manitou Springs, 745 P.2d 229 (Colo.)]