 BlackRock MuniHolding Insured Investm NYSE:MFL was the target of a large growth in short interest in March. As of March 13th, there was short interest totalling 8,600 shares, a growth of 207.1 from the February 27th total of 2,800 shares. Based on an average trading volume of 100,400 shares, the days-to-cover ratio is currently 0.1 ... 