目前relation extraction做到了哪一步？？总结下，下面只是给出了几篇相关文献还未总结
[coling2014]Relation classification via convolutional deep neural network
[emnlp2015] Distant supervision for relation extraction via piecewise convolutional neural networks
[acl2016] Attentionbased bidirectional long short-term memory networks for relation lassification.
[acl2016]Neural relation extraction with selective attention over instances
[emnlp2017]Adversarial Training for Relation Extraction