Broadway loves a backstage musical; something about going out a chorus girl and coming back a star appeals to the cheerleader in all of us. Stephen Sondheim had already written the lyrics for one of the greatest of them – Gypsy, in 1959 – when his friend and sometime collaborator James Goldman presented him with the idea for another. An Oscar winner for The Lion in Winter, Goldman was inspired by a newspaper item about a reunion of cast members from the Ziegfeld Follies. The Girls Upstairs, as the author called his new show, concerned just such a reunion, focusing on two exchorus girls and the stage-door Johnnies they married. Director Hal Prince had a different inspiration: a 1960 Life magazine photograph of silent-screen star Gloria Swanson, elegantly gowned, arms outstretched, standing in the rubble of the Roxy movie palace just before it was demolished. Thus The Girls Upstairs became Follies, a musical about two couples, certainly, but also about the end of an era, the end of opulence, the end of American innocence. At this reunion, the world-weary principals are shadowed and partnered by their exuberant younger selves. A brilliant, bounteous score featured Sondheim’s pitch-perfect homages to musical-theater titans from Romberg to Rodgers and Hammerstein, along with his own rather darker musings on love, marriage, and survival – songs like “I’m Still Here” and “Could I Leave You?” and “Too Many Mornings” that have become cabaret staples. For this first Broadway revival since the 1971 opening, the company itself comprises the best of the American theater, with Blythe Danner, Judith Ivey, Gregory Harrison, and Treat Williams in the principal roles. But also look for dance greats Marge Champion and Donald Saddler, and just try to keep your heart in its place when Joan Roberts – Oklahoma!’s original Laurey! – belts out “One More Kiss.” Talk about something to cheer for.