Conditional Random Fields (CRF) -- unfinished

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Reference:
1. John Lafferty, Andrew McCallum, Fernando Pereira. “Conditional Random Fields: Probabilistic Models for Segmenting and Labeling Sequence Data”, ICML2001.
2. Fei Sha, Fernando Pereira. “Shallow parsing with Conditional Random Fields”, HLT-NAACL 2003.

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