Silvio Ghilardi Enrica Nicolini Silvio Ranise Daniele Zucchelli Noetherianity and Combination Problems. 206-220 2007 conf/frocos/2007 FroCos http://dx.doi.org/10.1007/978-3-540-74621-8_14 db/conf/frocos/frocos2007.html#GhilardiNRZ07