Björk didn’t do the obvious thing with her boxed set, which would be to collect her hits along with the remixes that have made her a club goddess. Instead, ‘’Family Tree’’ is an idiosyncratic dissection of her own music. One disc contains 12 hits, in a slightly different selection than the one-disc ‘’Greatest Hits’’ album that was released separately this year. The box also contains five three-inch mini-CD’S of just four or five songs each, classified as Beats, Strings and Roots. They separate Björk’s songs into components : her first experiments with pulsating dance-floor electronics, her voice backed only by a string quartet, and, on Roots, assorted songs in Icelandic and English, some by Björk’s old band the Sugarcubes, one (“Generous Palmstroke”) an intense live performance accompanied only by a harp. The mini-CD’s expose Björk’s voice in all its vulnerability and power ; they also separate out the rock, classical, pop and dance elements in her songs. But the various threads of her music sound even better when they’re all tangled up.