Pattern from Scott Wlaschin (F#/.NET community) in which business logic is modeled as two parallel tracks: a successful ('upper') track and an error ('lower') track. Steps execute left-to-right on the success track; any failing step diverts execution to the error track, which short-circuits remaining success steps. Can be generalized to many tracks and many steps. Inspired Trailblazer 2.0's step DSL.