Marpurg's prelude to the chorale "Was mein GOtt will" is again a case of two compositions rolled into one. The first is a bicinium, where the choral melody is embelished with eigth notes in the right hand. Most of the notes of the choral melody are played in the right moment, Marpurg just writes three other eigth notes in between every note that belongs to the choral melody. The choral melody is clearly recognizable and the effect is rather playful.
The second half of the composition has the choral melody again in the soprano voice, this time in halve notes. The left hand takes care of the forward motion of the piece by playing a lot of eigth notes. The middle voice just fills in the blancs of the harmonies.
Score available here: http://partitura.org/index.php/friedrich-wilhelm-marpurg-was-mein-gott-will