For instance, a message can be regarded as harmless on its own, but when taking previous threads into account it may be seen as abusive, and vice versa. This aspect makes detecting abusive language extremely laborious even for human annotators; therefore it is difficult to build a large and reliable dataset BIBREF10 . 
 Question: What examples of the difficulties presented by the context-dependent nature of online aggression do they authors give?
detecting abusive language extremely laborious it is difficult to build a large and reliable dataset