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Where bindings can see evidence from the pattern match of the `GRHSs` they belong to, but not from anything in any of the guards (which belong to one of possibly many RHSs). Before this patch, we did *not* consider said evidence, causing #18533, where the lack of considering type information from a case pattern match leads to failure to resolve the vanilla COMPLETE set of a data type. Making available that information required a medium amount of refactoring so that `checkMatches` can return a `[(Deltas, NonEmpty Deltas)]`; one `(Deltas, NonEmpty Deltas)` for each `GRHSs` of the match group. The first component of the pair is the covered set of the pattern, the second component is one covered set per RHS. Fixes #18533. Regression test case: T18533
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