Steven Moffat on 'Sherlock' special; how long series can go

Executive producer Rebecca Eaton, co-writer/executive producer Steven Moffat and producer Sue Vertue speak onstage during the "Sherlock" panel at the PBS 2015 Summer TCA Tour held at the Beverly Hilton Hotel on Friday, July 31, 2015 in Beverly Hills, Calif. Photo by Richard Shotwell/Invision/AP