Read "C6/9" as a six-nine chord, not a slash bass

[matt-edmondson]

Fixes #248

The problem

Chord.TryParse treated everything after the first / as a slash-bass note, calling TryParseRoot on it — which requires the first character to be a note letter A–G. The six-nine symbol found on most jazz lead sheets (C6/9, Cm6/9, F6/9) is not a slash chord at all: the /9 stacks an added ninth on a sixth chord. "9" is not a note letter, so Chord.TryParse("C6/9") returned false and Chord.Parse("C6/9") threw FormatException — even though the package documents sixths, ninths and slash bass as supported.

The change

Semantics.Music/Chord.cs:

• TryReadRoot no longer fails outright when the post-slash token is not a note letter. It first asks TryRewriteSixNine whether the symbol is the six-nine idiom — a bare 9 directly after a 6.
• TryRewriteSixNine rewrites C6/9 to C6add9, the spelling ConsumeModifiers already understands. Routing through add9 rather than a bare 9 is what keeps the ninth an addition: a bare 9 would run through ApplyExtensions and imply a dominant seventh the chord does not have.
• The rewritten symbol is read from the top, so a real slash bass after the idiom still parses: C6/9/G is a six-nine over a G bass.

C6/9 parses to Sixth = Natural, Tensions |= Nine, Seventh = None, no bass — tones [0, 4, 7, 9, 14], which is Cadd9 plus the natural sixth.

The guard is deliberately narrow. A 9 not preceded by 6 (C/9), or one followed by another digit (C6/11), is still rejected, so no previously-invalid symbol other than the idiom starts parsing.

ToString() is unchanged and emits C6add9, which re-parses to an equal chord.

Tests

Semantics.Test/Music/ChordTests.cs — five new tests: the parse shape, the ChordTones() identity against Cadd9, the minor form, the idiom over a real slash bass, and the negative cases that must still fail.

Semantics.Test/Music/ChordRoundTripTests.cs — C6/9, Cm6/9 and C6/9/G added to the round-trip corpus.

Verified by reverting Chord.cs alone and rebuilding: 5 of the 23 chord tests fail (Parse_SixNine_IsAnAddedNinthOverASixth_NotASlashBass, ChordTones_SixNine_IsAdd9PlusTheNaturalSixth, Parse_MinorSixNine, Parse_SixNine_OverASlashBass, CanonicalOutputRoundTrips). Parse_SlashOverANonNoteStillFails passes either way by design — it pins the guard against over-broadening.

With the fix restored, the full suite is green: 1255 tests, 0 failed (8 skipped, Windows-only).

Semantics.Music builds clean across all five target frameworks, and the local SonarCloud analyzer pass documented in CLAUDE.md reports nothing new in the touched files.

Docs

Semantics.Music/README.md and the Chord summary now list the six-nine idiom among the supported symbols.

🤖 Generated with Claude Code

https://claude.ai/code/session_01MNK9bgoZL8ocxTSncTfDEp

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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