Mary John Miller, U.S. Treasury undersecretary for domestic finance, said at an industry conference that the five regulators working on the Volcker rule should produce a single version. "We think a joint rule is optimal," she said. "To have a different rule developed by a banking regulator and a securities regulator would not be helpful to the market ... That requires a significant amount of coordination and discussion to think about how they can all work together. And that process takes time -- it has taken time." Meanwhile, former Federal Reserve Chairman Paul Volcker says progress on the rule has been hindered by a fragmented regulatory landscape and other obstacles.