Akio Fujiyoshi Takumi Kasai Spinal-Formed Context-Free Tree Grammars. 59-83 2000 33 Theory Comput. Syst. 1 http://link.springer.de/link/service/journals/00224/bibs/0033001/00330059.html db/journals/mst/mst33.html#FujiyoshiK00