Ever since world music has been "discovered" by larger audiences, there has been a tension between purists and those who would muck around with traditional music. One performer who has embodied that family feud has been Natalie MacMaster. The Cape Breton fiddler fronts a rock-like band and has occasionally gone a bit afield from her homeland's Celtic tradition, but then has jumped back to playing it straight (the blonde performer playfully titled her last album My Roots Are Showing). This time out, MacMaster has it both ways with a double-CD set. On the first disc, she plays with her regular band and lets her ambitions run, stretching to include a flamenco-flavored tune and doing some step-dancing herself. On the second, she plays at a backwoods square dance in Cape Breton with just a pianist and a guitarist. The first disc is a good introduction to MacMaster's music, but the second disc is the winner here. Playing with metronomic steadiness, the trio never lets the dancers down and MacMaster's playing on the skeins of tunes she gathers is as lovely as it is propulsive.