Though their fusion of rock and classical set the stage for prog, Procol Harum always valued the concision of pop. That said, even their simplest songs are rife with rich detail: Soulful serenade “Salad Days (Are Here Again)” previews the moonbeamed organ sound they would master on “A Whiter Shade of Pale,” while “Souvenir in London” is an acoustic foot-stomper with player-piano-like melodies that make you feel you're in a bustling bordello.