U2 have made their current European tour a testing ground for new material. The best of the new live songs is "Mercy," cut during the sessions for 2004's How to Dismantle an Atomic Bomb but not on the album. The band has reworked "Mercy" for the stage, turning it into a big bear hug of a ballad that almost sounds like it could have gone on Joshua Tree. "You're going to kill me, and I want to die," Bono sings against the Edge's spacey arpeggios. "We were meant for each other." Other tracks that thrilled fans this summer include the intensely rocking "Return of the Stingray Guitar," the psychedelic "Glastonbury" and the starkly beautiful "Every Breaking Wave." The latter recalls "All I Want Is You," with Bono delivering dark poetry like "The waves know we're on the rocks/Drowning is no sin." U2 say they have four separate albums in the works; if so, this is a good start.