[johnml1135]

Quick summary

A literal phonological rule such as m -> p / x_ now unapplies during analysis when the phonemes carry no phonological features, so xpuma (the synthesis of xmuma) parses again instead of returning nothing.
The cause was the nonvacuous-unapplication guard in FeatureAnalysisRewriteRuleSpec: it looked only at symbolic features, and in a featureless table the only thing a literal rule changes is StrRep, a string feature.
Feature-defined segments keep their existing path; synthesis is untouched.

Where to look

• FeatureAnalysisRewriteRuleSpec: the inverse now restores the input StrRep (it previously kept the output spelling), and a changed string feature counts as nonvacuous. Pinned by StrRepRewriteRuleTests.LiteralRewriteWithoutFeaturesAnalyzesItsSynthesisAndRejectsVacuousUnapplication (xpuma -> xmuma; muma, xuma kept; xmuma, puma rejected; a second unapplication on the same word is vacuous).
• A rule after a segment class: t -> d / V_ with V={a} synthesizes kat as kad. LiteralRewriteAfterSegmentClassAnalyzesKadAndRejectsKat pins that kad analyzes to kat and that kat and five neighbours are rejected.
• StringFeatureValue.IsSupersetOf(StringFeatureValue): a public wrapper over the existing protected superset check, mirroring SymbolicFeatureValue.
• Both tests fail with the production change removed.

Deliberately not included

• A full local solution build: CMake is not installed on this machine, so ./local_check.sh --agent-strict stopped at SentencePiece's native build after hygiene, restore and formatting passed. CI covers it.
• 721c6043 shortens one pre-existing comment in ScrVersExtensions.cs that the comment-hygiene gate rejects on master. It is a separate commit with no behaviour change; drop it if it is unwanted here.

Validation

• dotnet test tests/SIL.Machine.Morphology.HermitCrab.Tests -- 108 passed, 0 failed.
• dotnet test ... --filter FullyQualifiedName~StrRepRewriteRuleTests with the production fix reverted -- both tests fail; restored, both pass.
• dotnet build src/SIL.Machine.Morphology.HermitCrab -c Release and src/SIL.Machine.Morphology.HermitCrab.Tool -c Release -- 0 warnings, 0 errors.
• dotnet csharpier check . -- clean.
• pwsh -NoProfile -File scripts/comment-hygiene.ps1 -- clean.
• ./local_check.sh --agent-strict -- not completed: CMake missing (see above).

Issue / porting context

Found while measuring FieldWorks projects with an underdefined phonology through HCLoader and HermitCrab (context in #537). In a FieldWorks project whose phonemes have no features, any literal-phoneme rewrite rule loses every analysis that needs it to unapply. PanGloss (the Rust port) carries the same fix and pins it with an oracle-diffed fixture.

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How it was found

A synthetic FieldWorks project defined m -> p / x_ with featureless phonemes. Exported through HCLoader 9.3.10 and run on Machine 3.8.2, xpuma had no analysis although xmuma synthesizes to it. A probe that replaced only the target pattern's Acceptable predicate with true made the target match and the existing x_ environment accept, which isolated the guard. Live HCLoader and the exported XML agreed on every word, so the loader is not involved.

🤖 Generated with Claude Code

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This change is 

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.43%. Comparing base (8983141) to head (960a710).

Files with missing lines	Patch %	Lines
...honologicalRules/FeatureAnalysisRewriteRuleSpec.cs	92.85%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
+ Coverage   74.38%   74.43%   +0.04%     
==========================================
  Files         456      456              
  Lines       38343    38360      +17     
  Branches     5264     5267       +3     
==========================================
+ Hits        28522    28552      +30     
+ Misses       8665     8643      -22     
- Partials     1156     1165       +9     

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[claude]

Verdict: approve. F3 is fixed. Since the last round the PR was rebased onto master and gained one new test, and I found no new defect. The production change still applies only to segments defined without features.

• Most important: nothing open. The last finding, F3 (FeatureAnalysisRewriteRuleSpec.cs:62), is now covered by StrRepRewriteRuleTests.cs:106.
• Counts: 0 Critical, 0 Important, 0 Low new.
• What I ran: dotnet test tests/SIL.Machine.Morphology.HermitCrab.Tests --filter FullyQualifiedName~StrRepRewriteRuleTests gave 5 passed, 0 failed. I also ran /code-review high. Its candidates repeat ones dropped in earlier rounds: the rhsAntiFSs index skew, string-feature variables, LINQ cost, the ScrVersExtensions.cs comment, and AnalysisMorphologicalTransform. For CI, gh pr checks 538 shows Build on ubuntu-22.04 passing and Build on windows-latest still pending. The advisory Comment hygiene check passes, but it does not stand in for --agent-strict, which I left to CI.
• Not verified: whether ModifyFromInput affix-process analysis has the same featureless StrRep defect (AnalysisMorphologicalTransform.cs:33). If it does, that is a follow-up, not part of this PR.

Public API: adds StringFeatureValue.IsSupersetOf(StringFeatureValue, bool) to SIL.Machine. No target framework, package, or machine.py parity contract changes.

History was rewritten since 960a710 (the compare reports diverged), so I reviewed the whole PR again.

Findings: F1 addressed (68cd30e), F2 addressed (960a710), F3 addressed (ebd3a38).

Reviewed at ebd3a38