Every so often, the schedules and whims of Little Rock’s musical lions align. Tonight, for example, when Greg Spradlin, Isaac Alexander, Jason Weinheimer and Chris Michaels, joined by drummer/songwriter Stephen Chopek (who charmed at an Undercroft concert last August), take the stage at the White Water Tavern as The Libras, a shifting collective who have interpreted the oeuvres of Neil Young, Elvis Costello, Tom Waits and a host of other boogie and rock pioneers over the years. Concerts from The Libras come around less often than Bud Light changes its can design, so get on out tonight if you’re able.