It works!

Here's the result of the giant previous post in 5 lines of a test:

    -- Trill transitions from 2 semitones to 1 semitone.
    equal (run
        [ ("*twelve", [(0, 0, "4a"), (4, 0, "i (4b)")])
        , ("t-diatonic", [(0, 0, "tr _ 1")])
        ])
        ([[(0, 69), (1, 71.25), (2, 70), (3, 71.75), (4, 71), (5, 72)]], [])

It's actually a little bit surprising when I try out the original motivation and what comes out is exactly what should.