José L. Balcázar Albert Bifet Antoni Lozano Mining Frequent Closed Unordered Trees Through Natural Representations. 347-359 2007 conf/iccs/2007 ICCS http://dx.doi.org/10.1007/978-3-540-73681-3_26 db/conf/iccs/iccs2007.html#BalcazarBL07