Jun Suzuki Tsutomu Hirao Yutaka Sasaki Eisaku Maeda Hierarchical Directed Acyclic Graph Kernel: Methods for Structured Natural Language Data. 32-39 2003 ACL http://acl.ldc.upenn.edu/acl2003/main/pdfs/Suzuki.pdf db/conf/acl/acl2003.html#SuzukiHSM03