Why the Indigos? Because after gigging and gigging, they've finally got a grip on the trick of rearranging familiar elements into something new. Both David Kay and Dick Smith write quietly mature songs that sound airy and gentle even when played loud. Kay's tunes in particular betray a kind of coffeehouse-folkie sensibility, so it's only fitting that he should be blessed with one of those breezy, urgent Phil Ochs-ish voices you seldom hear anymore, even in cofeehouses. Smith's numbers, on the other hand, inject more of a catchy, pop 'n' roll feel into the proceedings. As arrangers, the Indigos brandish the same jangly open-chorded guitar you've heard elsewhere, but it's their shy enthusiasm that keeps them from turning into just another boring midwest pop band. They practice a sturdy songcraft hat seems at once both naive and wise and doesn't call attention to its own cleverness. And yes, they can rock. Thursday, Lounge Ax, 2438 N. Lincoln; 525-6620.