Luca Tummolini Cristiano Castelfranchi Trace Signals: The Meanings of Stigmergy. 141-156 2006 conf/e4mas/2006 E4MAS http://dx.doi.org/10.1007/978-3-540-71103-2_8 db/conf/e4mas/e4mas2006.html#TummoliniC06