Timo Knuutila Magnus Steinby The Inference of Tree Languages from Finite Samples: An Algebraic Approach. 337-367 1994 129 Theor. Comput. Sci. 2 db/journals/tcs/tcs129.html#KnuutilaS94 http://dx.doi.org/10.1016/0304-3975(94)90033-7