n federal question cases where applicable federal substantive law exists, there is no federal statutory or common law of landlord and tenant. Powers I. United States Postal Serv., 671 F.2d 1041, 1042, 1045 (7th Cir.1982); Reed I. United States Postal Serv., 660 F.Supp. 178, 181 (D.Mass.1987). The question remains, therefore,