Level 1: reduction to simultaneities. In this case, notation of
domains would have been tedious.
Level 2: consolidation of octave doublings. In this case, the reduction removed most of the top voice, revealing a clear descending line, paralleled in the bass.
Level 3: reduction to underlying domains and prolongations.
Level 4: The middle domain is shown as a prolongation. Notice the underlying 1-5-1 bass.