Accompany Brahms’s Wiegenlied (Cradle Song), Op. 49 No. 4, while the app sings the melody. Keep the rocking accompaniment quiet and legato under the singer’s phrase, breathe with the voice at the ends of its lines and let the bass carry the harmony. A pedal point holds one bass note; the ground-bass card shows a whole bass line repeated under changing music, the frame of a passacaglia. At the keyboard: accompany a melody over a pedal point.