. Covariance in unsupervised learning of probabilistic grammars. Journal of Machine Learning Research, 11:3117-3151, 2010. CSZ06. Olivier Chapelk, Bernhard Schakopf, and Alexander Zien. Semi-Supervised Learning. MIT Press, 2006. CXYNI05. Yun Chi, Yi Xia, Yirong Yang, and Richard Ft Muntz. Mining closed and max