One might reasonably object that my systematic working through the proto-backgrounds has just delayed the obvious: the alto clearly moves from ^3, while the soprano is just as obviously based on ^5. That sanguine certainty, however, is undermined when one realizes that a proto-background ^3-^5 means the interval also holds sway at the end of the piece: in other words, this reading is radically anti-teleological. Only three of the nine possible proto-backgrounds support such "beginning-loaded" hearings: the unison ^3, the unison ^5, and ^3-^5.
The second level in the graphic below conveys this curiously static sense: the work unfolds in a leisurely way from a firm initial premise, enfolding the C# major section by means of a simple (chromatic) neighbor note and demoting the cadence (whether rising in its direct sense or falling in its hidden sense). The second level also conceals the reprise's instability, which is duly sorted out and explained in the details of the third level.
