Tomorrow, the Securities and Exchange Commission is expected to pass a new set of regulations for stock market trading in the US. Having pondered long and hard over how to reform the "trade through" rule that has favoured the New York Stock Exchange, William Donaldson, SEC chairman, looks set to extend the rule to Nasdaq, thus upsetting many institutional investors and Nasdaq itself.

There is nothing wrong with upsetting Wall Street per se, but Mr Donaldson is not justified in this case. He has dropped an earlier version of his plan that would have been even more dirigiste in determining on which market a trade is executed. But the remaining proposal is a solution in search of a problem. To extend rather than to abolish the trade through rule is going in the wrong direction.