Daiji Fukagawa Tatsuya Akutsu Atsuhiro Takasu Constant Factor Approximation of Edit Distance of Bounded Height Unordered Trees. 7-17 2009 SPIRE http://dx.doi.org/10.1007/978-3-642-03784-9_2 conf/spire/2009 db/conf/spire/spire2009.html#FukagawaAT09