With swagger and snarl, Girl In a Coma turn their sophomore album Trio B.C. into an affirmation of classic punk virtues. Not that this San Antonio-based combo insists on genre purity — there are bursts of twanging rockabilly (“B.B.”) and early ‘60s Latino rock (“Ven Cerca”) to leaven the obvious ‘70s and ‘80s influences. Lead singer/guitarist Nina Diaz pitches her vocals between a wail and a growl, and with sister Phanie on drums and Jenn Alva on bass, she bears down hard on spiky, angst-ridden rockers like “Baby Boy,” “Pleasure and Pain” and “Empty Promise.” The Coma gang gets frisky on the Jam-like “In the Day,” and waltzes with a rough-edged tenderness on “Vino.” Most surprising are the gentler moments, especially “El Monte,” a vision of domestic happiness accented with slide guitar and trumpet. Joan Jett (co-owner of the group’s label) makes a vocal appearance on “Joanie In the City,” a ferocious punk-feminist anthem. Trio B.C. displays a focus and consistency that far more experienced bands might envy.