Affiliation: | a Department of Mathematics, University of Washington, Seattle, Washington, 98195, f1E-mail: babson@math.washington.eduf1b Department of Mathematics, University of Massachusetts, Amherst, Massachusetts, 01003, f2E-mail: gunnells@math.umass.eduf2c Department of Mathematics and Computer Science, Santa Clara University, Santa Clara, California, 95053, f3E-mail: rscott@math.scu.eduf3 |