While relatively new, the seq2seq approach has achieved state-of-the-art results in not only its original application -machine translation - (Luong et al., 2015b; Jean et al., 2015a; Luong et al., 2015a; Jean et al., 2015b; Luong & Manning, 2015) , but also image caption generation , and constituency parsing<cite> (Vinyals et al., 2015a)</cite> .