@article{JMLR:v11:cohen10a,
  author  = {Shay B. Cohen and Noah A. Smith},
  title   = {Covariance in Unsupervised Learning of Probabilistic Grammars},
  journal = {Journal of Machine Learning Research},
  year    = {2010},
  volume  = {11},
  number  = {101},
  pages   = {3017--3051},
  url     = {http://jmlr.org/papers/v11/cohen10a.html}
}