Long short-term memories ("LSTMs": Hochreiter and Schmidhuber (1997)), a particular variant of RNN, have become particularly popular, and been successfully applied to a large number of tasks: speech recognition (Graves et al., 2013), sequence tagging (Huang et al., 2015), document categorisation (Yang et al., 2016), and machine translation .
although CRFs are adept at capturing local structure, the problem does not naturally suit a linear sequential structure, i.e.