Look to the shawls; let them show you the way. All night you’ve been anticipating their arrival on the Fleetwood Mac stage: the witchy moment when Stevie Nicks, that blonde chanteuse, abruptly disappears from view and, with a simple costume change she’s perfected over 35 years, reemerges a woman transformed, wrapped in fringed silk signaling a visitation by Rhiannon or Gold Dust Woman or the livid spurned lover of “Stand Back,” fine fabric unfurling from her delicate shoulders like the banner of an advancing army, heralding not just a song but the coming of an event. There may also be a wind machine, or perhaps you’re just imagining it. This was all to be expected, and somehow it still thrills. Twirling in the outstretched arms of Stevie Nicks, those shawls have magic in them.