TODO: recent paper about SPN: http://homes.cs.washington.edu/~pedrod/papers/uai11a.pdf
application in NLP: http://www.di.uniba.it/~ndm/pubs/vergari15ecml.pdf
TODO: recent paper about SPN: http://homes.cs.washington.edu/~pedrod/papers/uai11a.pdf
application in NLP: http://www.di.uniba.it/~ndm/pubs/vergari15ecml.pdf