Franz Baader Pavlos Marantidis Alexander Okhotin Approximately Solving Set Equations. 37-43 2016 UNIF http://users.mat.unimi.it/users/ghilardi/UNIF2016/UNIF16-abstracts.pdf#page=37 conf/unif/2016 db/conf/unif/unif2016.html#BaaderMO16