In the popular mind, a big part of what happens on Wall Street consists of jacketless middle-aged men in red braces smoking cigars as they thrash out big-money late-night deals. For that, thank Bruce Wasserstein, chief executive of Lazards, who died this week. His innovation and tactical brilliance helped mould the modern mergers and acquisitions industry. The corner of finance he carved out, however, is a mixed legacy.

There is such a thing as synergy: it is possible that two companies, when combined, can be worth much more than the sum of their parts. It can make sense for a company to bring some functions in-house, or to combine forces with those of a competitor to benefit from greater economies of scale.