Keith Emerson of Emerson Lake & Palmer in 1970. Chris Walter/WireImage/Getty For close to a half century, prog has been the breeding ground for rock’s most out-there, outsized and outlandish ideas: Thick-as-a-brick concept albums, an early embrace of synthesizers, overly complicated time signatures, Tolkienesque fantasies, travails from future days and scenes from a memory. In celebration of Rush’