After years of renting, King Arthur wanted to buy his castle, Camelot, outright but the bank was wary about lending him the money, fearing that he might default on payments. To persuade the bank that he was a safe risk, he offered the blade of his famous, priceless sword, Excalibur, as collateral. The bank agreed to these terms but a year later, with financial markets in disarray, it began to get twitchy and demanded the whole of Excalibur as collateral. King Arthur was distraught. "You can't su…