onsanto Co, 558 F.2d 257, 263-64 (5th Cir. 1977). Among the circumstances which must be considered is "the purpose for which intervention is sought."National Resources Defense Council v. Costle 561 F.2d 904, 907 (D.C.Cir. 1977). Mr. Epstein's motion is timely because it seeks intervention only for the limited purpose of challeng