“…689 We also emphasize that features 2, 7, 9, 10-24, 30-31 have not been explored before, being thus new contributions of this 690 work. Other features, notably features 32, 33, 38, 41, 55, 60, 65, 67, 69-71, 73-79, 81, 90-125 have been adapted from similar 691 concepts in other domains (e.g., question-answering sites, Wikipedia, and YouTube) [1,11,12,30,36]. 692 …”