ag's prelude to the chorale "Lobe den Herren" has the same construction as his prelude to "Wir Christenleut (see here: https://partitura.org/index.php/christian-gotthilf-tag-wir-christenleut/). The chorale melody is not used as a cantus firmus, or a theme for a fugue, it is not used as bassline in the pedal's, or any of the more familiar forms of choral prelude. Tag uses this melody to create a virtuosic piece, where the repetitions in the chorale melody itself are taken a step further by giving each fragment of the choral melody an echo like repetition. Interspersed with passage work it gives the impression of being a stand alone piece. You have to be very familiar with the chorale melody to be able to recognize it in the music.