Deciding equivalence with sums and the empty type

The logical technique of focusing can be applied to the $λ$-calculus; in a simple type system with atomic types and negative type formers (functions, products, the unit type), its normal forms coincide with $βη$-normal forms. Introducing a saturation phase gives a notion of quasi-normal forms in presence of positive types (sum types and the empty type). This rich structure let us prove the decidability of $βη$-equivalence in presence of the empty type, the fact that it coincides with contextual equivalence, and a finite model property.

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