Complete, as in "two." But it's fine that famed British modernist Harrison Birtwistle waited until his 60s to start writing these. "The Tree of Strings," finished in 2007, is first up, and earns its half-hour running time. Though undeniably complex in nature, the single-movement piece makes its transitions from whisper-quiet harmonics to jaunty atonal jabbings with considerable elegance. "Movements for String Quartet," an earlier piece, is a more casual collection -- more like a sketchbook than "Tree of Strings." Don't say old modernists can't learn new tricks.