Conditional Random Field Compressed sentences have to remain grammatical, which implies a dependence between values in y.
without any restrictions on the dependence between choices, it is intractable to marginalize over all possible y with a scoring function for a given (x, y) pair, we assume that only neighboring values in y are dependent, and apply a linear-chain CRF (Lafferty et al., 2001) that uses h as its features to this problem.