Some have focused on reducing the number of UNKs by enabling NMT to learn from a larger vocabulary (Jean et al., 2015;Mi et al., 2016); others have focused on replacing UNKs by copying source words (Gulcehre et al., 2016;Gu et al., 2016;Luong et al., 2015b).
these methods only help with unknown words, not rare words.