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Rap these days is about abstraction , about mythical rags-to-riches stories , impossibly hard characters rising to the top of the world by staying hard , by never talking to police , by being smarter and tougher than the other guy . Any lawyer will tell you these perjury cases are notoriously hard to prove . The performance analysis of multiserver systems is notoriously hard , especially when the system involves resource sharing or prioritization . " And it 's extraordinarily hard to do that . Second , scientists soon rediscovered that the problem is surprisingly hard . So it 's hard to catch up with - - and certainly hard to surpass - - the public 's fancy for tinkering with educational models . Enforcing employment contracts is notoriously hard to do .