Are you trolling or what. Let's see what's written in the comment above. > If A reduces to B, it means that an algorithm implementing B can be used (with some

Are you trolling or what. Let's see what's written in the comment above. > If A reduces to B, it means that an algorithm implementing B can be used (with some pre- and post-processing) to solve A. Here we say that that we can solve a "hard problem" if we can express it in terms of the "Document Ranking" problem. Let's rewrite that quoted sentence: An algorithm implementing "Document Ranking" can be used (with some pre- and post-processing) to solve "Hard problem". Let's do substitution in the first part of the sentence, "If A reduces to B", where A is "hard problem" and B is "Document Ranking": Hard problem reduces to Document Ranking. That means EXACTLY that we can USE Document Ranking to SOLVE the Hard Problem. Just as we wanted.