Senator Tom Coburn had what he thought was irrefutable evidence. Goldman Sachs Group Inc. trader Josh Birnbaum had recommended betting against the stock of Bear Stearns Cos. in July 2007, just four months after his colleagues sold a $300m piece of “one [explicative deleted] deal” to hedge funds controlled by Bear Stearns, according to e-mails obtained by the Senate through a subpoena. To Coburn, an Oklahoma Republican, that meant Goldman Sachs was wagering against its own clients using inside information — the poor quality of the deal it sold Bear Stearns.