This week L.A. Weekly published its “Top 20 Singer-Songwriters of All Time.” It’s yet another impossible list. For starters, how do you define it? As artists who write and perform primarily as a solo act? That may explain the omission of Lennon and McCartney. Or, as I often think of the term, artists who largely draw from the acoustic, roots and country origins. But that doesn’t explain Bowie and Prince. And nothing explains the inclusion of Dolly Parton and the exclusion of Jackson Browne. Or this man. Guy Clark died today at age 74, a Texas legend forever encased in leather, whiskey and cigarette smoke.