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The `PmExprEq` business was a huge hack and was at the same time vastly too powerful and not powerful enough to encode negative term equalities, i.e. facts of the form "forall y. x ≁ Just y". This patch introduces the concept of 'refutable shapes': What matters for the pattern match checker is being able to encode knowledge of the kind "x can no longer be the literal 5". We encode this knowledge in a `PmRefutEnv`, mapping a set of newly introduced `PmAltCon`s (which are just `PmLit`s at the moment) to each variable denoting above inequalities. So, say we have `x ≁ 42 ∈ refuts` in the term oracle context and try to solve an equality like `x ~ 42`. The entry in the refutable environment will immediately lead to a contradiction. This machinery renders the whole `PmExprEq` and `ComplexEq` business unnecessary, getting rid of a lot of (mostly dead) code. See the Note [Refutable shapes] in TmOracle for a place to start. Metric Decrease: T11195
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