Davide Sottara Paola Mello Mark Proctor Towards Modelling Defeasible Reasoning with Imperfection in Production Rule Systems. 345-352 2009 RuleML http://dx.doi.org/10.1007/978-3-642-04985-9_32 conf/ruleml/2009 db/conf/ruleml/ruleml2009.html#SottaraMP09