The idea of Crosby, Stills, Nash, and Young is one that would typically work better as a concept than as a musical reality, but not in this case. Prior to the band’s formation, initially as a trio before Young was brought into the operation by Atlantic Records’ Ahmet Ertegun, all four members were accomplished in their own right as songwriters, even if this didn’t quite qualify them as a supergroup. While their talent was unquestionable, not man…