[1
]DISP, Università di Roma Tor Vergata, via del Politecnico 1, 00133 Rome, Italy
[2
]Dept. of Computer Science, University of Maryland, A.V. Williams Bldg. 115, College
Park 20742, MD, USA
[3
]Fraunhofer Center for Experimental Software Engineering Maryland, College Park, Maryland,
20742