By the time the Securities and Exchange Commission finalized a rule last month to regulate derivatives under the Dodd-Frank financial reform law, the big banks that dominate the multitrillion-dollar market had already figured out how to game it.

This is not a tale, however, of how wily banks always find a way around the rules. In this case, the S.E.C. has written and passed a rule that is custom built for evasion, all the while insisting, unconvincingly, that it does not have the legal authority to be any tougher.

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