Why no Wall St. bigwig has been prosecuted

``Consider an oft-cited example of a high Wall Street crime: Lehman Brothers' use of an accounting tactic known as Repo 105. Lehman exploited a loophole in accounting rules to sell assets, getting them off its balance sheet even as it was obligated to repurchase them a short while later. This gave it an end-of-quarter appearance of being less leveraged... he seeming intent--to mislead investors--was dishonest, and the tactic raised concerns within Lehman at the time. To prosecute it criminally, however, you would need to show willful intent to defraud, which is difficult.''