In this 2013 file photo, partygoers smoke marijuana during a Prohibition-era themed New Year's Eve party at a bar in Denver, celebrating the start of retail pot sales. Denver is starting work on becoming the first city in the nation to allow marijuana clubs and public pot use in places like restaurants, yoga studios and art galleries. Voters narrowly approved the "social use" measure last November. (AP Photo/Brennan Linsley, File)