1. Department of Mathematics; Georgetown University; Washington DC; 20057-1233; U.S.A.
2. Department of Statistics; University of Missouri-Columbia; 146 Middlebush Hall; Columbia; MO; 65211-6100; U.S.A.
3. US Geological Survey, Columbia Environmental Research Center; 4200 New Haven Road; Columbia; MO; 65201; U.S.A.