Yun Chi Yi Xia Yirong Yang Richard R. Muntz Mining Closed and Maximal Frequent Subtrees from Databases of Labeled Rooted Trees. 190-202 2005 17 IEEE Trans. Knowl. Data Eng. 2 http://dx.doi.org/10.1109/TKDE.2005.30 db/journals/tkde/tkde17.html#ChiXYM05