Surafel Lemma Abebe Sonia Haiduc Andrian Marcus Paolo Tonella Giuliano Antoniol Analyzing the Evolution of the Source Code Vocabulary. 189-198 2009 CSMR http://dx.doi.org/10.1109/CSMR.2009.61 conf/csmr/2009 db/conf/csmr/csmr2009.html#AbebeHMTA09