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Chord round-trip breaks for a seventh with add11/add13: "C7add13" prints as "C713", which reparses as C13 with a 9th and 11th #300
Description
What's wrong
Since #293 (ad24784), Chord.Parse accepts add11 and add13 after a seventh. For example, "C7add13" parses as a dominant seventh with tensions {13} only, and a check gives tones [0,4,7,10,21].
The formatter can't write that chord back. AppendTensions (Semantics.Music/Chord.cs:598-622) writes addN only when there is no seventh. With a seventh it writes the highest natural tension as a bare stacked number ("13", "11", "9"). On reparse, ApplyExtensions expands 13 into 9 + 11 + 13.
Observed (scratch app referencing Semantics.Music)
| Input | ToString() |
Reparsed tones | Equal |
|---|---|---|---|
C7add13 |
C713 |
[0,4,7,10,14,17,21] vs original [0,4,7,10,21] |
False |
C7add11 |
C711 |
gains a 9th | False |
C9add13 |
C713 |
gains an 11th | False |
Cmaj7add13 |
Cmaj713 |
gains a 9th and 11th | False |
The round-trip corpus (ChordRoundTripTests.cs:12-18) has no seventh+addN case, so the tests don't catch this.
Suggested fix
When there is a seventh:
- Write the highest contiguous natural stack as a number: 9 →
9, 9+11 →11, 9+11+13 →13. - Write each remaining natural tension that isn't implied by that number as
addN. For example,{13}gives7add13and{9,13}gives9add13.
The alternative is to reject addN after a seventh in the parser again, but that would undo #293.
Acceptance criteria
C7add13, C7add11, C9add13 and Cmaj7add13 are added to the round-trip corpus, and each one round-trips to equal tones.
Activity
matt-edmondson commentedon Sep 28, 2026
Triage
ToString()of a valid chord reparses as a different chord with extra tones, so any caller that stores chords as text silently corrupts them. Only seventh chords withadd11oradd13are affected.Semantics.Music/Chord.cs(AppendTensions, lines 598-622) andChordRoundTripTests.csNotes: The suggested fix is right: emit the highest contiguous natural stack as a number, then
addNfor the rest. Keeping #293 is better than rejectingaddNagain. Add the four failing inputs to the round-trip corpus first, so the fix is test-driven.Generated by Claude Code
matt-edmondson commentedon Sep 30, 2026
Covered by #360, which redesigns this bug cluster as a whole. Implement it through that issue rather than individually.
Generated by Claude Code
[minor] Redesign chord symbols; fixes #300, #322, #336, #340, #341, #344