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The object of T B L is to learn an ordered sequence of transformation rules. Such rules dictate when - based on the context - a word should have its tag changed. An example would be "replace tag vb with nn if the word immediately to the left has a tag dr." Here is how this rule is represented in the # - T B L rule/template formalism: | Proceedings of EACL 99 X-TBL Lite A Small Extendible Transformation-Based Learner Torbjorn Lager Department of Linguistics Uppsala University Sweden Torb j orn. Lager@ling.uu.se Abstract This short paper describes - and in fact gives the complete source for - a tiny Prolog program implementing a flexible and fairly efficient TransformationBased Learning TBL system 1 Introduction Transformation-Based Learning Brill 1995 is a well-established learning method in NLP circles. This short paper presents a light version of the ẶÍ-TBL system - a general logically transparent flexible and efficient transformation-based learner presented in Lager 1999 . It turns out that a transformation-based learner complete with a compiler for templates can be implemented in less than one page of Prolog code. 2 Z-TBL Rules Representations The point of departure for TBL is a tagged initialstate corpus and a correctly tagged training corpus. Assuming the part-of-speech tagging task corpus data can be represented by means of three kinds of clauses wd P W is true iff the word w is at position p in the corpus tag P A is true iff the word at position p in the corpus is tagged A tag A B P is true iff the word at p is tagged A and the correct tag for the word at p is B Although this representation may seem a bit redundant it provides exactly the kind of indexing into the data that is needed.1 A decent Prolog system can deal with millions of such clauses. Assuming a Prolog with first argument indexing. The p-TBL systems are implemented in SICStus Prolog. The object of TBL is to learn an ordered sequence of transformation rules. Such rules dictate when - based on the context - a word should have its tag changed. An example would be replace tag vb with nn if the word immediately to the left has a tag dt. Here is how this rule is represented in the p-TBL rule template formalism tag vb nn - tag dtQ -l . Conditions may refer to different features and complex conditions may be composed from simpler .
