Department of Computer Science, Iowa State University, Ames 50011, USA. wcchang@iastate.edu|Department of Computer Science, Iowa State University, Ames 50011, USA. wcchang@iastate.edu|Department of Computer Science, Iowa State University, Ames 50011, USA. wcchang@iastate.edu|Department of Computer Science, Iowa State University, Ames 50011, USA. wcchang@iastate.edu