added missing abstract method
added missing abstract method

jackwilsdon/beets,beetbox/beets,shamangeorge/beets,sampsyo/beets,beetbox/beets,ibmibmibm/beets,shamangeorge/beets,jackwilsdon/beets,shamangeorge/beets,jackwilsdon/beets,jackwilsdon/beets,sampsyo/beets,ibmibmibm/beets,shamangeorge/beets,sampsyo/beets,ibmibmibm/beets,sampsyo/beets,beetbox/beets,ibmibmibm/beets,beetbox/beets