Attempts to be exhaustive on backward-compatibility coverage to raise confidence that future refactors will not introduce unintended regressions.
Each incompatible condition is tested separately by comparing two spec files where the only difference is that fine-grained incompatible condition.
In some cases the current behavior appears incorrect. Tests are still added for these to avoid unintended regression, but are given TODO comments for later follow-up.
Pre-existing tests have been removed if they are redundant to avoid confusion and to follow the convention.
Closes #545